# Class HashTree

java.lang.Object
org.apache.jorphan.collections.HashTree

All Implemented Interfaces:
  `Serializable, Cloneable, Map<Object,HashTree>`

Direct Known Subclasses:
  `ListedHashTree, SortedHashTree`

public class HashTree extends Object implements Serializable, Map<Object,HashTree>, Cloneable
This class is used to create a tree structure of objects. Each element in the tree is also a key to the next node down in the tree. It provides many ways to add objects and branches, as well as many ways to retrieve.

HashTree implements the Map interface for convenience reasons. The main difference between a Map and a HashTree is that the HashTree organizes the data into a recursive tree structure, and provides the means to manipulate that structure.

Of special interest is the `traverse(HashTreeTraverser)` method, which provides an expedient way to traverse any HashTree by implementing the `HashTreeTraverser` interface in order to perform some operation on the tree, or to extract information from the tree.

See Also:
  - `HashTreeTraverser`
  - `SearchByClass`
  - Serialized Form

## Nested Class Summary

### Nested classes/interfaces inherited from interface Map

`Map.Entry<K,V>`

## Field Summary

Fields
Modifier and Type
Field
Description
`protected final Map<Object,HashTree>`
`data`

## Constructor Summary

Constructors
Modifier
Constructor
Description
``
`HashTree()`
Creates an empty new HashTree.
``
`HashTree(Object key)`
Creates a new HashTree and adds the given object as a top-level node.
``
`HashTree(Object[] keys)`
Creates a new HashTree and adds all the objects in the given array as top-level nodes in the tree.
``
`HashTree(Collection<?> keys)`
Creates a new HashTree and adds all the objects in the given collection as top-level nodes in the tree.
`protected`
`HashTree(Map<Object,HashTree> _map)`
Allow subclasses to provide their own Map.

## Method Summary

Modifier and Type
Method
Description
`HashTree`
`add(Object key)`
Adds an key into the HashTree at the current level.
`void`
`add(Object[] keys)`
Adds all the given objects as nodes at the current level.
`HashTree`
`add(Object[] treePath, Object value)`
`void`
`add(Object[] treePath, Object[] values)`
Adds a series of nodes into the HashTree using the given path.
`void`
`add(Object[] treePath, Collection<?> values)`
Adds a series of nodes into the HashTree using the given path.
`HashTree`
`add(Object key, Object value)`
Adds a key and it's value in the HashTree.
`void`
`add(Object key, Object[] values)`
Adds a key and it's values in the HashTree.
`void`
`add(Object key, Collection<?> values)`
Adds a key as a node at the current level and then adds all the objects in the second argument as nodes of the new node.
`void`
`add(Object key, HashTree subTree)`
Adds a key as a node at the current level and then adds the given HashTree to that new node.
`void`
`add(Collection<?> keys)`
Adds a bunch of keys into the HashTree at the current level.
`HashTree`
`add(Collection<?> treePath, Object value)`
Adds a series of nodes into the HashTree using the given path.
`void`
`add(Collection<?> treePath, Object[] values)`
Adds a series of nodes into the HashTree using the given path.
`void`
`add(Collection<?> treePath, Collection<?> values)`
Adds a series of nodes into the HashTree using the given path.
`void`
`add(HashTree newTree)`
Adds all the nodes and branches of the given tree to this tree.
`protected HashTree`
`addTreePath(Collection<?> treePath)`
`void`
`clear()`
Clears the HashTree of all contents.
`Object`
`clone()`
Create a clone of this HashTree.
`protected void`
`cloneTree(HashTree newTree)`
`boolean`
`containsKey(Object o)`
If the HashTree contains the given object as a key at the top level, then a true result is returned, otherwise false.
`boolean`
`containsValue(Object value)`
Implemented as required by the Map interface, but is not very useful here.
`protected HashTree`
`createNewTree()`
Creates a new tree.
`protected HashTree`
`createNewTree(Object key)`
Creates a new tree.
`protected HashTree`
`createNewTree(Collection<?> values)`
Creates a new tree.
`Set<Map.Entry<Object,HashTree>>`
`entrySet()`
Exists to satisfy the Map interface.
`boolean`
`equals(Object o)`
Compares all objects in the tree and verifies that the two trees contain the same objects at the same tree levels.
`HashTree`
`get(Object key)`
Returns the HashTree object associated with the given key.
`Object[]`
`getArray()`
Gets an array of all keys in the current HashTree node.
`Object[]`
`getArray(Object key)`
Gets an array of all keys in the HashTree mapped to the given key of the current HashTree object (in other words, one level down).
`Object[]`
`getArray(Object[] treePath)`
Recurses down into the HashTree stucture using each subsequent key in the array of keys, and returns an array of keys of the HashTree object at the end of the recursion.
`Object[]`
`getArray(Collection<?> treePath)`
Recurses down into the HashTree structure using each subsequent key in the treePath argument, and returns an array of keys of the HashTree object at the end of the recursion.
`HashTree`
`getTree(Object key)`
Gets the HashTree mapped to the given key.
`HashTree`
`getTree(Object[] treePath)`
Gets the HashTree object mapped to the last key in the array by recursing through the HashTree structure one key at a time.
`HashTree`
`getTree(Collection<?> treePath)`
Gets the HashTree object mapped to the last key in the SortedSet by recursing through the HashTree structure one key at a time.
`protected HashTree`
`getTreePath(Collection<?> treePath)`
`int`
`hashCode()`
Returns a hashcode for this HashTree.
`boolean`
`isEmpty()`
If the HashTree is empty, true is returned, false otherwise.
`Set<Object>`
`keySet()`
Returns a Set of all the keys in the top-level of this HashTree.
`Collection<Object>`
`list()`
Gets a Collection of all keys in the current HashTree node.
`Collection<?>`
`list(Object key)`
Gets a Set of all keys in the HashTree mapped to the given key of the current HashTree object (in other words, one level down.
`Collection<?>`
`list(Object[] treePath)`
Recurses down into the HashTree stucture using each subsequent key in the array of keys, and returns the Set of keys of the HashTree object at the end of the recursion.
`Collection<?>`
`list(Collection<?> treePath)`
Recurses down into the HashTree stucture using each subsequent key in the List of keys, and returns the Set of keys of the HashTree object at the end of the recursion.
`HashTree`
`put(Object key, HashTree value)`
This is the same as calling HashTree.add(key,value).
`void`
`putAll(Map<? extends Object, ? extends HashTree> map)`
The Map given must also be a HashTree, otherwise an UnsupportedOperationException is thrown.
`HashTree`
`remove(Object key)`
Removes the entire branch specified by the given key.
`void`
`replaceKey(Object currentKey, Object newKey)`
Finds the given current key, and replaces it with the given new key.
`HashTree`
`search(Object key)`
Searches the HashTree structure for the given key.
`void`
`set(Object[] treePath, Object[] values)`
Sets a series of keys into the HashTree.
`void`
`set(Object[] treePath, Collection<?> values)`
Sets a series of keys into the HashTree.
`void`
`set(Object key, Object value)`
Sets a key and it's value in the HashTree.
`void`
`set(Object key, Object[] values)`
Sets a key and its values in the HashTree.
`void`
`set(Object key, Collection<?> values)`
Sets a key and its values in the HashTree.
`void`
`set(Object key, HashTree t)`
Sets a key into the current tree and assigns it a HashTree as its subtree.
`void`
`set(Collection<?> values)`
Sets the nodes of the current tree to be the objects of the given collection.
`void`
`set(Collection<?> treePath, Object[] values)`
Sets a series of keys into the HashTree.
`void`
`set(Collection<?> treePath, Collection<?> values)`
Sets a series of keys into the HashTree.
`int`
`size()`
Returns the number of top-level entries in the HashTree.
`String`
`toString()`
Generate a printable representation of the tree.
`void`
`traverse(HashTreeTraverser visitor)`
Allows any implementation of the HashTreeTraverser interface to easily traverse (depth-first) all the nodes of the HashTree.
`Collection<HashTree>`
`values()`
Returns a collection of all the sub-trees of the current tree.

### Methods inherited from class Object

`finalize, getClass, notify, notifyAll, wait, wait, wait`

### Methods inherited from interface Map

`compute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, merge, putIfAbsent, remove, replace, replace, replaceAll`

## Field Details

### data

protected final Map<Object,HashTree> data

## Constructor Details

### HashTree

public HashTree()
Creates an empty new HashTree.

### HashTree

protected HashTree(Map<Object,HashTree> _map)
Allow subclasses to provide their own Map.

Parameters:
  `_map` - `Map` to use

### HashTree

public HashTree(Object key)
Creates a new HashTree and adds the given object as a top-level node.

Parameters:
  `key` - name of the new top-level node

### HashTree

public HashTree(Collection<?> keys)
Creates a new HashTree and adds all the objects in the given collection as top-level nodes in the tree.

Parameters:
  `keys` - a collection of objects to be added to the created HashTree.

### HashTree

public HashTree(Object[] keys)
Creates a new HashTree and adds all the objects in the given array as top-level nodes in the tree.

Parameters:
  `keys` - array with names for the new top-level nodes

## Method Details

### putAll

public void putAll(Map<? extends Object, ? extends HashTree> map)
The Map given must also be a HashTree, otherwise an UnsupportedOperationException is thrown. If it is a HashTree, this is like calling the add(HashTree) method.

Specified by:
  `putAll` in interface `Map<Object,HashTree>`
See Also:
  - `add(HashTree)`
  - `Map.putAll(Map)`

### entrySet

public Set<Map.Entry<Object,HashTree>> entrySet()
Exists to satisfy the Map interface.

Specified by:
  `entrySet` in interface `Map<Object,HashTree>`
See Also:
  - `Map.entrySet()`

### containsValue

public boolean containsValue(Object value)
Implemented as required by the Map interface, but is not very useful here. All 'values' in a HashTree are HashTree's themselves.

Specified by:
  `containsValue` in interface `Map<Object,HashTree>`
Parameters:
  `value` - Object to be tested as a value.
Returns:
  True if the HashTree contains the value, false otherwise.
See Also:
  - `Map.containsValue(Object)`

### put

public HashTree put(Object key, HashTree value)
This is the same as calling HashTree.add(key,value).

Specified by:
  `put` in interface `Map<Object,HashTree>`
Parameters:
  `key` - to use
  `value` - to store against key
See Also:
  - `Map.put(Object, Object)`

### clear

public void clear()
Clears the HashTree of all contents.

Specified by:
  `clear` in interface `Map<Object,HashTree>`
See Also:
  - `Map.clear()`

### values

public Collection<HashTree> values()
Returns a collection of all the sub-trees of the current tree.

Specified by:
  `values` in interface `Map<Object,HashTree>`
See Also:
  - `Map.values()`

### add

public void add(Object key, HashTree subTree)
Adds a key as a node at the current level and then adds the given HashTree to that new node.

Parameters:
  `key` - key to create in this tree
  `subTree` - sub tree to add to the node created for the first argument.

### add

public void add(HashTree newTree)
Adds all the nodes and branches of the given tree to this tree. Is like merging two trees. Duplicates are ignored.

Parameters:
  `newTree` - the tree to be added

### containsKey

public boolean containsKey(Object o)
If the HashTree contains the given object as a key at the top level, then a true result is returned, otherwise false.

Specified by:
  `containsKey` in interface `Map<Object,HashTree>`
Parameters:
  `o` - Object to be tested as a key.
Returns:
  True if the HashTree contains the key, false otherwise.
See Also:
  - `Map.containsKey(Object)`

### isEmpty

public boolean isEmpty()
If the HashTree is empty, true is returned, false otherwise.

Specified by:
  `isEmpty` in interface `Map<Object,HashTree>`
Returns:
  True if HashTree is empty, false otherwise.

### set

public void set(Object key, Object value)
Sets a key and it's value in the HashTree. It actually sets up a key, and then creates a node for the key and sets the value to the new node, as a key. Any previous nodes that existed under the given key are lost.

Parameters:
  `key` - key to be set up
  `value` - value to be set up as a key in the secondary node

### set

public void set(Object key, HashTree t)
Sets a key into the current tree and assigns it a HashTree as its subtree. Any previous entries under the given key are removed.

Parameters:
  `key` - key to be set up
  `t` - HashTree that the key maps to

### set

public void set(Object key, Object[] values)
Sets a key and its values in the HashTree. It sets up a key in the current node, and then creates a node for that key, and sets all the values in the array as keys in the new node. Any keys previously held under the given key are lost.

Parameters:
  `key` - Key to be set up
  `values` - Array of objects to be added as keys in the secondary node

### set

public void set(Object key, Collection<?> values)
Sets a key and its values in the HashTree. It sets up a key in the current node, and then creates a node for that key, and set all the values in the array as keys in the new node. Any keys previously held under the given key are removed.

Parameters:
  `key` - key to be set up
  `values` - Collection of objects to be added as keys in the secondary node

### set

public void set(Object[] treePath, Object[] values)
Sets a series of keys into the HashTree. It sets up the first object in the key array as a key in the current node, recurses into the next HashTree node through that key and adds the second object in the array. Continues recursing in this manner until the end of the first array is reached, at which point all the values of the second array are set as keys to the bottom-most node. All previous keys of that bottom-most node are removed.

Parameters:
  `treePath` - array of keys to put into HashTree
  `values` - array of values to be added as keys to bottom-most node

### set

public void set(Object[] treePath, Collection<?> values)
Sets a series of keys into the HashTree. It sets up the first object in the key array as a key in the current node, recurses into the next HashTree node through that key and adds the second object in the array. Continues recursing in this manner until the end of the first array is reached, at which point all the values of the Collection of values are set as keys to the bottom-most node. Any keys previously held by the bottom-most node are lost.

Parameters:
  `treePath` - array of keys to put into HashTree
  `values` - Collection of values to be added as keys to bottom-most node

### set

public void set(Collection<?> treePath, Object[] values)
Sets a series of keys into the HashTree. It sets up the first object in the key list as a key in the current node, recurses into the next HashTree node through that key and adds the second object in the list. Continues recursing in this manner until the end of the first list is reached, at which point all the values of the array of values are set as keys to the bottom-most node. Any previously existing keys of that bottom node are removed.

Parameters:
  `treePath` - collection of keys to put into HashTree
  `values` - array of values to be added as keys to bottom-most node

### set

public void set(Collection<?> values)
Sets the nodes of the current tree to be the objects of the given collection. Any nodes previously in the tree are removed.

Parameters:
  `values` - Collection of objects to set as nodes.

### set

public void set(Collection<?> treePath, Collection<?> values)
Sets a series of keys into the HashTree. It sets up the first object in the key list as a key in the current node, recurses into the next HashTree node through that key and adds the second object in the list. Continues recursing in this manner until the end of the first list is reached, at which point all the values of the Collection of values are set as keys to the bottom-most node. Any previously existing keys of that bottom node are lost.

Parameters:
  `treePath` - list of keys to put into HashTree
  `values` - collection of values to be added as keys to bottom-most node

### add

public HashTree add(Object key)
Adds an key into the HashTree at the current level. If a HashTree exists for the key already, no new tree will be added

Parameters:
  `key` - key to be added to HashTree
Returns:
  newly generated tree, if no tree was found for the given key; existing key otherwise

### add

public void add(Object[] keys)
Adds all the given objects as nodes at the current level.

Parameters:
  `keys` - Array of Keys to be added to HashTree.

### add

public void add(Collection<?> keys)
Adds a bunch of keys into the HashTree at the current level.

Parameters:
  `keys` - Collection of Keys to be added to HashTree.

### add

public HashTree add(Object key, Object value)
Adds a key and it's value in the HashTree. The first argument becomes a node at the current level, and the second argument becomes a node of it.

Parameters:
  `key` - key to be added
  `value` - value to be added as a key in the secondary node
Returns:
  HashTree for which `value` is the key

### add

public void add(Object key, Object[] values)
Adds a key and it's values in the HashTree. The first argument becomes a node at the current level, and adds all the values in the array to the new node.

Parameters:
  `key` - key to be added
  `values` - array of objects to be added as keys in the secondary node

### add

public void add(Object key, Collection<?> values)
Adds a key as a node at the current level and then adds all the objects in the second argument as nodes of the new node.

Parameters:
  `key` - key to be added
  `values` - Collection of objects to be added as keys in the secondary node

### add

public void add(Object[] treePath, Object[] values)
Adds a series of nodes into the HashTree using the given path. The first argument is an array that represents a path to a specific node in the tree. If the path doesn't already exist, it is created (the objects are added along the way). At the path, all the objects in the second argument are added as nodes.

Parameters:
  `treePath` - an array of objects representing a path
  `values` - array of values to be added as keys to bottom-most node

### add

public void add(Object[] treePath, Collection<?> values)
Adds a series of nodes into the HashTree using the given path. The first argument is an array that represents a path to a specific node in the tree. If the path doesn't already exist, it is created (the objects are added along the way). At the path, all the objects in the second argument are added as nodes.

Parameters:
  `treePath` - an array of objects representing a path
  `values` - collection of values to be added as keys to bottom-most node

### add

public HashTree add(Object[] treePath, Object value)

### add

public void add(Collection<?> treePath, Object[] values)
Adds a series of nodes into the HashTree using the given path. The first argument is a List that represents a path to a specific node in the tree. If the path doesn't already exist, it is created (the objects are added along the way). At the path, all the objects in the second argument are added as nodes.

Parameters:
  `treePath` - a list of objects representing a path
  `values` - array of values to be added as keys to bottom-most node

### add

public HashTree add(Collection<?> treePath, Object value)
Adds a series of nodes into the HashTree using the given path. The first argument is a List that represents a path to a specific node in the tree. If the path doesn't already exist, it is created (the objects are added along the way). At the path, the object in the second argument is added as a node.

Parameters:
  `treePath` - a list of objects representing a path
  `value` - Object to add as a node to bottom-most node
Returns:
  HashTree for which `value` is the key

### add

public void add(Collection<?> treePath, Collection<?> values)
Adds a series of nodes into the HashTree using the given path. The first argument is a SortedSet that represents a path to a specific node in the tree. If the path doesn't already exist, it is created (the objects are added along the way). At the path, all the objects in the second argument are added as nodes.

Parameters:
  `treePath` - a SortedSet of objects representing a path
  `values` - Collection of values to be added as keys to bottom-most node

### addTreePath

protected HashTree addTreePath(Collection<?> treePath)

### getTree

public HashTree getTree(Object key)
Gets the HashTree mapped to the given key.

Parameters:
  `key` - Key used to find appropriate HashTree()
Returns:
  the HashTree for `key`

### get

public HashTree get(Object key)
Returns the HashTree object associated with the given key. Same as calling `getTree(Object)`.

Specified by:
  `get` in interface `Map<Object,HashTree>`
See Also:
  - `Map.get(Object)`

### getTree

public HashTree getTree(Object[] treePath)
Gets the HashTree object mapped to the last key in the array by recursing through the HashTree structure one key at a time.

Parameters:
  `treePath` - array of keys.
Returns:
  HashTree at the end of the recursion.

### clone

public Object clone()
Create a clone of this HashTree. This is not a deep clone (ie, the contents of the tree are not cloned).

Overrides:
  `clone` in class `Object`

### cloneTree

protected void cloneTree(HashTree newTree)

### createNewTree

protected HashTree createNewTree()
Creates a new tree. This method exists to allow inheriting classes to generate the appropriate types of nodes. For instance, when a node is added, it's value is a HashTree. Rather than directly calling the HashTree() constructor, the createNewTree() method is called. Inheriting classes should override these methods and create the appropriate subclass of HashTree.

Returns:
  HashTree

### createNewTree

protected HashTree createNewTree(Object key)
Creates a new tree. This method exists to allow inheriting classes to generate the appropriate types of nodes. For instance, when a node is added, it's value is a HashTree. Rather than directly calling the HashTree() constructor, the createNewTree() method is called. Inheriting classes should override these methods and create the appropriate subclass of HashTree.

Parameters:
  `key` - object to use as the key for the top level
Returns:
  newly created `HashTree`

### createNewTree

protected HashTree createNewTree(Collection<?> values)
Creates a new tree. This method exists to allow inheriting classes to generate the appropriate types of nodes. For instance, when a node is added, it's value is a HashTree. Rather than directly calling the HashTree() constructor, the createNewTree() method is called. Inheriting classes should override these methods and create the appropriate subclass of HashTree.

Parameters:
  `values` - objects to be added to the new `HashTree`
Returns:
  newly created `HashTree`

### getTree

public HashTree getTree(Collection<?> treePath)
Gets the HashTree object mapped to the last key in the SortedSet by recursing through the HashTree structure one key at a time.

Parameters:
  `treePath` - Collection of keys
Returns:
  HashTree at the end of the recursion

### list

public Collection<Object> list()
Gets a Collection of all keys in the current HashTree node. If the HashTree represented a file system, this would be like getting a collection of all the files in the current folder.

Returns:
  Set of all keys in this HashTree

### list

public Collection<?> list(Object key)
Gets a Set of all keys in the HashTree mapped to the given key of the current HashTree object (in other words, one level down. If the HashTree represented a file system, this would like getting a list of all files in a sub-directory (of the current directory) specified by the key argument.

Parameters:
  `key` - key used to find HashTree to get list of
Returns:
  Set of all keys in found HashTree.

### remove

public HashTree remove(Object key)
Removes the entire branch specified by the given key.

Specified by:
  `remove` in interface `Map<Object,HashTree>`
See Also:
  - `Map.remove(Object)`

### list

public Collection<?> list(Object[] treePath)
Recurses down into the HashTree stucture using each subsequent key in the array of keys, and returns the Set of keys of the HashTree object at the end of the recursion. If the HashTree represented a file system, this would be like getting a list of all the files in a directory specified by the treePath, relative from the current directory.

Parameters:
  `treePath` - Array of keys used to recurse into HashTree structure
Returns:
  Set of all keys found in end HashTree

### list

public Collection<?> list(Collection<?> treePath)
Recurses down into the HashTree stucture using each subsequent key in the List of keys, and returns the Set of keys of the HashTree object at the end of the recursion. If the HashTree represented a file system, this would be like getting a list of all the files in a directory specified by the treePath, relative from the current directory.

Parameters:
  `treePath` - List of keys used to recurse into HashTree structure
Returns:
  Set of all keys found in end HashTree

### replaceKey

public void replaceKey(Object currentKey, Object newKey)
Finds the given current key, and replaces it with the given new key. Any tree structure found under the original key is moved to the new key.

Parameters:
  `currentKey` - name of the key to be replaced
  `newKey` - name of the new key

### getArray

public Object[] getArray()
Gets an array of all keys in the current HashTree node. If the HashTree represented a file system, this would be like getting an array of all the files in the current folder.

Returns:
  array of all keys in this HashTree.

### getArray

public Object[] getArray(Object key)
Gets an array of all keys in the HashTree mapped to the given key of the current HashTree object (in other words, one level down). If the HashTree represented a file system, this would like getting a list of all files in a sub-directory (of the current directory) specified by the key argument.

Parameters:
  `key` - key used to find HashTree to get list of
Returns:
  array of all keys in found HashTree

### getArray

public Object[] getArray(Object[] treePath)
Recurses down into the HashTree stucture using each subsequent key in the array of keys, and returns an array of keys of the HashTree object at the end of the recursion. If the HashTree represented a file system, this would be like getting a list of all the files in a directory specified by the treePath, relative from the current directory.

Parameters:
  `treePath` - array of keys used to recurse into HashTree structure
Returns:
  array of all keys found in end HashTree

### getArray

public Object[] getArray(Collection<?> treePath)
Recurses down into the HashTree structure using each subsequent key in the treePath argument, and returns an array of keys of the HashTree object at the end of the recursion. If the HashTree represented a file system, this would be like getting a list of all the files in a directory specified by the treePath, relative from the current directory.

Parameters:
  `treePath` - list of keys used to recurse into HashTree structure
Returns:
  array of all keys found in end HashTree

### getTreePath

protected HashTree getTreePath(Collection<?> treePath)

### hashCode

public int hashCode()
Returns a hashcode for this HashTree.

Specified by:
  `hashCode` in interface `Map<Object,HashTree>`
Overrides:
  `hashCode` in class `Object`
See Also:
  - `Object.hashCode()`

### equals

public boolean equals(Object o)
Compares all objects in the tree and verifies that the two trees contain the same objects at the same tree levels. Returns true if they do, false otherwise.

Specified by:
  `equals` in interface `Map<Object,HashTree>`
Overrides:
  `equals` in class `Object`
Parameters:
  `o` - Object to be compared against
See Also:
  - `Object.equals(Object)`

### keySet

public Set<Object> keySet()
Returns a Set of all the keys in the top-level of this HashTree.

Specified by:
  `keySet` in interface `Map<Object,HashTree>`
See Also:
  - `Map.keySet()`

### search

public HashTree search(Object key)
Searches the HashTree structure for the given key. If it finds the key, it returns the HashTree mapped to the key. If it finds nothing, it returns null.

Parameters:
  `key` - Key to search for
Returns:
  HashTree mapped to key, if found, otherwise `null`

### size

public int size()
Returns the number of top-level entries in the HashTree.

Specified by:
  `size` in interface `Map<Object,HashTree>`
See Also:
  - `Map.size()`

### traverse

public void traverse(HashTreeTraverser visitor)
Allows any implementation of the HashTreeTraverser interface to easily traverse (depth-first) all the nodes of the HashTree. The Traverser implementation will be given notification of each node visited.

Parameters:
  `visitor` - the visitor that wants to traverse the tree
See Also:
  - `HashTreeTraverser`

### toString

public String toString()
Generate a printable representation of the tree.

Overrides:
  `toString` in class `Object`
Returns:
  a representation of the tree