JDK之java.util.Arrays

功能

JDK1.2起提供的工具类，实现对数组的排序、查找、复制等操作

源码

Sorting methods

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

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

Swaps x[a] with x[b]

private static void swap(Object[] x, int a, int b) {
    Object t = x[a];
    x[a] = x[b];
    x[b] = t;
}

Searching

public static int binarySearch(long[] a, long key) {
    return binarySearch0(a, 0, a.length, key);
}

public static int binarySearch(long[] a, int fromIndex, int toIndex,
                               long key) {
    rangeCheck(a.length, fromIndex, toIndex);
    return binarySearch0(a, fromIndex, toIndex, key);
}

Equality Testing

public static boolean equals(long[] a, long[] a2) {
    if (a==a2)
        return true;
    if (a==null || a2==null)
        return false;

    int length = a.length;
    if (a2.length != length)
        return false;

    for (int i=0; i<length; i++)
        if (a[i] != a2[i])
            return false;

    return true;
}

Filling

public static void fill(long[] a, long val) {
    // 定义变量len，执行效率高点
    for (int i = 0, len = a.length; i < len; i++)
        a[i] = val;
}
 
public static void fill(long[] a, int fromIndex, int toIndex, long val) {
    rangeCheck(a.length, fromIndex, toIndex);
    for (int i = fromIndex; i < toIndex; i++)
        a[i] = val;
}

toString

public static String toString(long[] 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(", ");
    }
}

asList

// JAVA5 可变参数
 @SafeVarargs
 @SuppressWarnings("varargs")
 public static <T> List<T> asList(T... a) {
     return new ArrayList<>(a);
 }

• 注意，asList返回的list不支持添加操作

public static void main(String[] args) {
    // 类型类：java.util.Arrays$ArrayList
    List stooges = Arrays.asList("Larry", "Moe", "Curly");
    // 不可添加元素 java.lang.UnsupportedOperationException
    // stooges.add("ee");
    // 类型类：java.util.ArrayList
    ArrayList<String> list = new ArrayList<>(stooges);
    list.add("diligent");
    // JDK5增强for循环
    for(String t : list){
        System.out.println(t);
    }
    System.out.println(list.size());
}

Cloning

@SuppressWarnings("unchecked")
public static <T> T[] copyOf(T[] original, int newLength) {
    return (T[]) copyOf(original, newLength, original.getClass());
}

public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
    @SuppressWarnings("unchecked")
    T[] copy = ((Object)newType == (Object)Object[].class)
        ? (T[]) new Object[newLength]
        : (T[]) Array.newInstance(newType.getComponentType(), newLength);
    System.arraycopy(original, 0, copy, 0,
                     Math.min(original.length, newLength));
    return copy;
}