深入了解Java核心类库--Arrays类

目录

• Java常用类库Arrays
• 一、常用方法
• 1.1 toString
• 1.2 Sort
• 1.2.1 sort​(T[] a, int fromIndex, int toIndex)
• 1.2.2 Sort(T[] a)
• 1.2.3 其它
• 1.3 copyOf
• 1.4 mismatch
• 1.5 binarySearch
• 1.5.1 binarySearch​(T[] a, int fromIndex, int toIndex, T key)
• 1.5.2 binarySearch​(T[] a, T key)
• 1.5.3 其它
• 1.6 equals
• 1.6.1 equals​(T[] a, int aFromIndex, int aToIndex, T[] b, int bFromIndex, int bToIndex)
• 1.6.2 equals​(T[] a, T[] a2)
• 1.6.3 其它
• 1.7 fills
• 1.7.1 fill​(T[] a, int fromIndex, int toIndex, T val)
• 1.7.2 fill​(T[] a, T val)
• 二、其他方法
• 总结

Java常用类库Arrays

类Arrays包含用于操作数组的各种方法（例如排序和搜索）

• 如果指定的数组引用为null，则此类中的方法都抛出NullPointerException ，除非另有说明

一、常用方法

1.1 toString

返回指定数组内容的字符串形式

举例

       int[] a1 = {1,2,3,4,5};
       System.out.println(Arrays.toString(a1));//[1, 2, 3, 4, 5]

源码

	public static String toString(int[] a) {
        if (a == null)
            return "null";
        int iMax = a.length - 1;
        if (iMax == -1)
            return "[]";
        StringBuilder b = new StringBuilder();
        b.append('[');
        for (int i = 0; ; i++) {
            b.append(a[i]);
            if (i == iMax)
                return b.append(']').toString();
            b.append(", ");
        }
    }

其它

1.2 Sort

排序（默认升序）

1.2.1 sort​(T[] a, int fromIndex, int toIndex)

指定区间进行排序

举例

       int[] a1 = {9,1,3,7,2,5};
       System.out.println(Arrays.toString(a1));//[9, 1, 3, 7, 2, 5]
       Arrays.sort(a1,0,3);//[0,3),对9,1,3进行排序
       System.out.println(Arrays.toString(a1));//[0[1, 3, 9, 7, 2, 5]

源码

    public static void sort(int[] a, int fromIndex, int toIndex) {
        rangeCheck(a.length, fromIndex, toIndex);
        DualPivotQuicksort.sort(a, fromIndex, toIndex - 1, null, 0, 0);
    }

1.2.2 Sort(T[] a)

对整个数组进行排序

举例

       int[] a1 = {0,7,8,2,4,1};
       System.out.println(Arrays.toString(a1));//[0, 7, 8, 2, 4, 1]
       Arrays.sort(a1);
       System.out.println(Arrays.toString(a1));//[0, 1, 2, 4, 7, 8]

源码

    public static void sort(int[] a) { DualPivotQuicksort.sort(a, 0, a.length - 1, null, 0, 0);
    }

1.2.3 其它

1.3 copyOf

复制（常用于数组扩容）

举例

       int[] a = {1,2,3};
       System.out.println(a.length);//output：3
       a = Arrays.copyOf(a,15);
       System.out.println(a.length);//output：15

源码

  public static int[] copyOf(int[] original, int newLength) {
        int[] copy = new int[newLength];
        System.arraycopy(original, 0, copy, 0, Math.min(original.length, newLength));
        return copy;
    }

其它

1.4 mismatch

举例

       int[] a1 = {0,1,2,3,4,5};
       int[] a2 = {0,1,2,3,4,5};//与a1相同
       int[] a3 = {0,1,2,3,0,5};//从索引4开始与a1不同
       System.out.println(Arrays.mismatch(a1,a2));//output：-1
       System.out.println(Arrays.mismatch(a1,a3));//output：4

源码

 public static int mismatch(int[] a, int[] b) {
        int length = Math.min(a.length, b.length); // Check null array refs
        if (a == b)
            return -1;
        int i = ArraysSupport.mismatch(a, b, length);
        return (i < 0 && a.length != b.length) ? length : i;
    }

其它

1.5 binarySearch

二分查找，搜索，返回下标

1.5.1 binarySearch​(T[] a, int fromIndex, int toIndex, T key)

限定了搜索的范围[fromIndex, toIndex）

举例

  int[] a = {1,2,3,4,5};
       int x1 = Arrays.binarySearch(a,2,3,4);//在a数组下标[2,3)中查找值为4的下标
       System.out.println(x1);//output：<0的随机数

源码

	public static int binarySearch(int[] a, int fromIndex, int toIndex,int key) {
        rangeCheck(a.length, fromIndex, toIndex);
        return binarySearch0(a, fromIndex, toIndex, key);
    }
    private static int binarySearch0(int[] a, int fromIndex, int toIndex,int key) {
        int low = fromIndex;
        int high = toIndex - 1;
        while (low <= high) {
            int mid = (low + high) >>> 1;
            int midVal = a[mid];
            if (midVal < key)
                low = mid + 1;
            else if (midVal > key)
                high = mid - 1;
            else
                return mid; // key found
        }
        return -(low + 1);  // key not found.
    }

1.5.2 binarySearch​(T[] a, T key)

与上述相同，只是没有限定范围，fromIndex=0, toIndex=length

举例

       int[] a = {1,2,3,4,5};
       int x1 = Arrays.binarySearch(a,3);//在a数组中查找值为3的下标
       int x2 = Arrays.binarySearch(a,-6);//在a数组中查找值为6的下标
       System.out.println(x1);//output：2
       System.out.println(x2);//output：<0的随机数

源码

    public static int binarySearch(int[] a, int key) {
        return binarySearch0(a, 0, a.length, key);
    }
	private static int binarySearch0(int[] a, int fromIndex, int toIndex,int key) {
        int low = fromIndex;
        int high = toIndex - 1;
        while (low <= high) {
            int mid = (low + high) >>> 1;
            int midVal = a[mid];
            if (midVal < key)
                low = mid + 1;
            else if (midVal > key)
                high = mid - 1;
            else
                return mid; // key found
        }
        return -(low + 1);  // key not found.
    }

1.5.3 其它

1.6 equals

1.6.1 equals​(T[] a, int aFromIndex, int aToIndex, T[] b, int bFromIndex, int bToIndex)

如果两个指定数组在指定范围内相等 ，则返回true

举例

       int[] a1 = {1,2,3,4,5};
       int[] a2 = {1,2,0,0,4,5};
       System.out.println(Arrays.equals(a1,0,2,a2,0,2));//true
       System.out.println(Arrays.equals(a1,3,5,a2,4,6));//true

源码

public static boolean equals(int[] a, int aFromIndex, int aToIndex,int[] b, int bFromIndex, int bToIndex) {
        rangeCheck(a.length, aFromIndex, aToIndex);
        rangeCheck(b.length, bFromIndex, bToIndex);
        int aLength = aToIndex - aFromIndex;
        int bLength = bToIndex - bFromIndex;
        if (aLength != bLength)
            return false;
        return ArraysSupport.mismatch(a, aFromIndex, b, bFromIndex,aLength) < 0;
    }

1.6.2 equals​(T[] a, T[] a2)

如果两个指定数组相等，则返回 true

举例

       int[] a1 = {1,2,3,4,5};
       int[] a2 = {1,2,3,4,5};
       int[] a3 = {1,2,0,4,5};
       System.out.println(Arrays.equals(a1,a2));//true
       System.out.println(Arrays.equals(a1,a3));//false

源码

 public static boolean equals(int[] a, int[] a2) {
        if (a==a2)
            return true;
        if (a==null || a2==null)
            return false;
        int length = a.length;
        if (a2.length != length)
            return false;
        return ArraysSupport.mismatch(a, a2, length) < 0;
    }

1.6.3 其它

1.7 fills

1.7.1 fill​(T[] a, int fromIndex, int toIndex, T val)

将指定的T值分配给指定的T类型数组的指定范围的每个元素

举例

       int[] a1 = new int[10];
       Arrays.fill(a1,1,4,8);
       char[] a2 = new char[10];
       Arrays.fill(a2,0,3,'s');
       System.out.println(Arrays.toString(a1));//[0, 8, 8, 8, 0, 0, 0, 0, 0, 0]
       System.out.println(Arrays.toString(a2));//[s, s, s,  ,  ,  ,  ,  ,  ,  ]

源码

    public static void fill(char[] a, int fromIndex, int toIndex, char val) {
        rangeCheck(a.length, fromIndex, toIndex);
        for (int i = fromIndex; i < toIndex; i++)
            a[i] = val;
    }

1.7.2 fill​(T[] a, T val)

将指定的T值分配给指定的T类型数组的每个元素

举例

       int[] a1 = new int[10];
       Arrays.fill(a1,8);
       char[] a2 = new char[10];
       Arrays.fill(a2,'s');
       System.out.println(Arrays.toString(a1));//[8, 8, 8, 8, 8, 8, 8, 8, 8, 8]
       System.out.println(Arrays.toString(a2));//[s, s, s, s, s, s, s, s, s, s]

源码

	public static void fill(int[] a, int val) {
        for (int i = 0, len = a.length; i < len; i++)
            a[i] = val;
    }

二、其他方法

总结

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