Java ArrayList

時間 2019-11-11

標籤 java arraylist 欄目 Java 简体版

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Java ArrayList的構造方法和方法html

Constructor Summary

Constructors
Constructor	Description
`ArrayList()`	Constructs an empty list with an initial capacity of ten.
`ArrayList(int initialCapacity)`	Constructs an empty list with the specified initial capacity.
`ArrayList(Collection<? extendsE> c)`	Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.

Method Summary

All MethodsInstance MethodsConcrete Methods
Modifier and Type	Method	Description
`void`	`add(int index,E element)`	Inserts the specified element at the specified position in this list.
`boolean`	`add(E e)`	Appends the specified element to the end of this list.
`boolean`	`addAll(int index,Collection<? extendsE> c)`	Inserts all of the elements in the specified collection into this list, starting at the specified position.
`boolean`	`addAll(Collection<? extends E> c)`	Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's Iterator.
`void`	`clear()`	Removes all of the elements from this list.
`Object`	`clone()`	Returns a shallow copy of this `ArrayList` instance.
`boolean`	`contains(Object o)`	Returns `true` if this list contains the specified element.
`void`	`ensureCapacity(int minCapacity)`	Increases the capacity of this `ArrayList` instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.
`void`	`forEach(Consumer<? super E> action)`	Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
`E`	`get(int index)`	Returns the element at the specified position in this list.
`int`	`indexOf(Object o)`	Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.
`boolean`	`isEmpty()`	Returns `true` if this list contains no elements.
`Iterator<E>`	`iterator()`	Returns an iterator over the elements in this list in proper sequence.
`int`	`lastIndexOf(Object o)`	Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element.
`ListIterator<E>`	`listIterator()`	Returns a list iterator over the elements in this list (in proper sequence).
`ListIterator<E>`	`listIterator(int index)`	Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list.
`E`	`remove(int index)`	Removes the element at the specified position in this list.
`boolean`	`remove(Object o)`	Removes the first occurrence of the specified element from this list, if it is present.
`boolean`	`removeAll(Collection<?> c)`	Removes from this list all of its elements that are contained in the specified collection.
`boolean`	`removeIf(Predicate<? super E> filter)`	Removes all of the elements of this collection that satisfy the given predicate.
`protected void`	`removeRange(int fromIndex, int toIndex)`	Removes from this list all of the elements whose index is between `fromIndex`, inclusive, and `toIndex`, exclusive.
`boolean`	`retainAll(Collection<?> c)`	Retains only the elements in this list that are contained in the specified collection.
`E`	`set(int index,E element)`	Replaces the element at the specified position in this list with the specified element.
`int`	`size()`	Returns the number of elements in this list.
`Spliterator<E>`	`spliterator()`	Creates a late-binding and fail-fast `Spliterator` over the elements in this list.
`List<E>`	`subList(int fromIndex, int toIndex)`	Returns a view of the portion of this list between the specified `fromIndex`, inclusive, and `toIndex`, exclusive.
`Object[]`	`toArray()`	Returns an array containing all of the elements in this list in proper sequence (from first to last element).
`<T> T[]`	`toArray(T[] a)`	Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array.
`void`	`trimToSize()`	Trims the capacity of this `ArrayList` instance to be the list's current size.

解析ArrayList的構造函數java

// 默認構造函數，默認容量大小爲10
ArrayList()

// capacity是ArrayList的默認容量大小。每次擴容爲原來的1.5倍。
ArrayList(int capacity)

// 建立一個包含collection的ArrayList,能夠將別的ArrayList數組複製進去
ArrayList(Collection<? extends E> collection)

ArrayList適合查詢與修改，格局位置索引index查詢和修改只須要花費常數時間api

get(int index)數組

set(int index, E element)oracle

可是對ArrayList的插入，刪除是很是耗時的，除非是在數組末端函數

add(int index,E element)this

remove(int index)//刪除索引位置的元素spa

remove(Obiect o)//尋找等於 o 的元素，其實就是每一個元素都比較一下，因此若是是自定義類就必需重寫equals方法code

因此咱們知道了ArrayList不適合頻繁插入刪除的場景，適合根據索引頻繁查詢修改的場景orm

ArrayList轉數組

toArray()//默認返回一個Object數組（注意！不能夠(String[])toArray()，你只能對一個個元素轉型不能對一個數組總體轉型，因此建議使用第二種方法）

toArray(T[] a)//本身設置數組類型好比toArray(new String[list.size()])就是返回一個數組大小=list長度元素類型爲String的數組

ArrayList擴容