System.arraycopy
- 只有数组为一维数组且元素为基本类型、String 类型的时候是深拷贝，其它情况下都属于浅拷贝，比如元素是引用类型、二维数组的情况
- 调用的是 native 方法，性能好
- 需要传入 dest
- 可以指定起始位置和拷贝的长度，比较灵活
/**
* Copies an array from the specified source array, beginning at the
* specified position, to the specified position of the destination array.
* A subsequence of array components are copied from the source
* array referenced by <code>src</code> to the destination array
* referenced by <code>dest</code>. The number of components copied is
* equal to the <code>length</code> argument. The components at
* positions <code>srcPos</code> through
* <code>srcPos+length-1</code> in the source array are copied into
* positions <code>destPos</code> through
* <code>destPos+length-1</code>, respectively, of the destination
* array.
* <p>
* If the <code>src</code> and <code>dest</code> arguments refer to the
* same array object, then the copying is performed as if the
* components at positions <code>srcPos</code> through
* <code>srcPos+length-1</code> were first copied to a temporary
* array with <code>length</code> components and then the contents of
* the temporary array were copied into positions
* <code>destPos</code> through <code>destPos+length-1</code> of the
* destination array.
* <p>
* If <code>dest</code> is <code>null</code>, then a
* <code>NullPointerException</code> is thrown.
* <p>
* If <code>src</code> is <code>null</code>, then a
* <code>NullPointerException</code> is thrown and the destination
* array is not modified.
* <p>
* Otherwise, if any of the following is true, an
* <code>ArrayStoreException</code> is thrown and the destination is
* not modified:
* <ul>
* <li>The <code>src</code> argument refers to an object that is not an
* array.
* <li>The <code>dest</code> argument refers to an object that is not an
* array.
* <li>The <code>src</code> argument and <code>dest</code> argument refer
* to arrays whose component types are different primitive types.
* <li>The <code>src</code> argument refers to an array with a primitive
* component type and the <code>dest</code> argument refers to an array
* with a reference component type.
* <li>The <code>src</code> argument refers to an array with a reference
* component type and the <code>dest</code> argument refers to an array
* with a primitive component type.
* </ul>
* <p>
* Otherwise, if any of the following is true, an
* <code>IndexOutOfBoundsException</code> is
* thrown and the destination is not modified:
* <ul>
* <li>The <code>srcPos</code> argument is negative.
* <li>The <code>destPos</code> argument is negative.
* <li>The <code>length</code> argument is negative.
* <li><code>srcPos+length</code> is greater than
* <code>src.length</code>, the length of the source array.
* <li><code>destPos+length</code> is greater than
* <code>dest.length</code>, the length of the destination array.
* </ul>
* <p>
* Otherwise, if any actual component of the source array from
* position <code>srcPos</code> through
* <code>srcPos+length-1</code> cannot be converted to the component
* type of the destination array by assignment conversion, an
* <code>ArrayStoreException</code> is thrown. In this case, let
* <b><i>k</i></b> be the smallest nonnegative integer less than
* length such that <code>src[srcPos+</code><i>k</i><code>]</code>
* cannot be converted to the component type of the destination
* array; when the exception is thrown, source array components from
* positions <code>srcPos</code> through
* <code>srcPos+</code><i>k</i><code>-1</code>
* will already have been copied to destination array positions
* <code>destPos</code> through
* <code>destPos+</code><i>k</I><code>-1</code> and no other
* positions of the destination array will have been modified.
* (Because of the restrictions already itemized, this
* paragraph effectively applies only to the situation where both
* arrays have component types that are reference types.)
*
* @param src the source array.
* @param srcPos starting position in the source array.
* @param dest the destination array.
* @param destPos starting position in the destination data.
* @param length the number of array elements to be copied.
* @exception IndexOutOfBoundsException if copying would cause
* access of data outside array bounds.
* @exception ArrayStoreException if an element in the <code>src</code>
* array could not be stored into the <code>dest</code> array
* because of a type mismatch.
* @exception NullPointerException if either <code>src</code> or
* <code>dest</code> is <code>null</code>.
*/
//src 源数组
//srcPos 源数组起始位置
//dest 目标数组
//destPos 目标数组起始位置
//length 要拷贝的长度（要拷贝的元素数量）
public static native void arraycopy(Object src, int srcPos,
Object dest, int destPos,
int length);
测试
String[] array = {"a", "b", "c", "b", "d"};
System.out.println("array.length:" + array.length);
String[] arrayCopy=new String[10];
System.out.println("arrayCopy.length:" + arrayCopy.length);
System.arraycopy(array,1,arrayCopy,1,4);
System.out.println(Arrays.toString(array));
System.out.println(Arrays.toString(arrayCopy));
array[2] = "cc";
System.out.println(Arrays.toString(array));
System.out.println(Arrays.toString(arrayCopy));
//-------------------------- 结果 ---------------------
array.length:5
arrayCopy.length:10
[a, b, c, b, d]
[null, b, c, b, d, null, null, null, null, null]
[a, b, cc, b, d]
[null, b, c, b, d, null, null, null, null, null]
Arrays.copyOf
- 调用了 System.arraycopy 方法
- 适合目标数组不明确的情况下使用，自动创建新的数组
/**
* Copies the specified array, truncating or padding with nulls (if necessary)
* so the copy has the specified length. For all indices that are
* valid in both the original array and the copy, the two arrays will
* contain identical values. For any indices that are valid in the
* copy but not the original, the copy will contain <tt>null</tt>.
* Such indices will exist if and only if the specified length
* is greater than that of the original array.
* The resulting array is of exactly the same class as the original array.
*
* @param <T> the class of the objects in the array
* @param original the array to be copied
* @param newLength the length of the copy to be returned
* @return a copy of the original array, truncated or padded with nulls
* to obtain the specified length
* @throws NegativeArraySizeException if <tt>newLength</tt> is negative
* @throws NullPointerException if <tt>original</tt> is null
* @since 1.6
*/
@SuppressWarnings("unchecked")
public static <T> T[] copyOf(T[] original, int newLength) {
return (T[]) copyOf(original, newLength, original.getClass());
}
/**
* Copies the specified array, truncating or padding with nulls (if necessary)
* so the copy has the specified length. For all indices that are
* valid in both the original array and the copy, the two arrays will
* contain identical values. For any indices that are valid in the
* copy but not the original, the copy will contain <tt>null</tt>.
* Such indices will exist if and only if the specified length
* is greater than that of the original array.
* The resulting array is of the class <tt>newType</tt>.
*
* @param <U> the class of the objects in the original array
* @param <T> the class of the objects in the returned array
* @param original the array to be copied
* @param newLength the length of the copy to be returned
* @param newType the class of the copy to be returned
* @return a copy of the original array, truncated or padded with nulls
* to obtain the specified length
* @throws NegativeArraySizeException if <tt>newLength</tt> is negative
* @throws NullPointerException if <tt>original</tt> is null
* @throws ArrayStoreException if an element copied from
* <tt>original</tt> is not of a runtime type that can be stored in
* an array of class <tt>newType</tt>
* @since 1.6
*/
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
@SuppressWarnings("unchecked")
//创建一个新的长度为 newLength 的数组实例
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
//进行拷贝，如果长度不足，意味着后面的保留默认值，此时是 null
//Math.min 存在一定量的开销
System.arraycopy(original, 0, copy, 0,
Math.min(original.length, newLength));
return copy;
}
测试
String[] array = {"a", "b", "c", "b", "d"};
System.out.println("array.length:" + array.length);
String[] arrayCopyed = Arrays.copyOf(array, 10);
System.out.println("arrayCopyed.length:" + arrayCopyed.length);
System.out.println(Arrays.toString(array));
System.out.println(Arrays.toString(arrayCopyed));
array[2] = "cc";
System.out.println(Arrays.toString(array));
System.out.println(Arrays.toString(arrayCopyed));
//-------------------------- 结果 ---------------------
array.length:5
arrayCopyed.length:10
[a, b, c, b, d]
[a, b, c, b, d, null, null, null, null, null]
[a, b, cc, b, d]
[a, b, c, b, d, null, null, null, null, null]
