Java Integer为什么要使用equals而不会==比较

先看一段代码

package com.lang;

/**
 * @author wangyl-910
 * @description
 * @date 2017/11/4
 * @see
 */
public class Integer {
    public static void main(String[] args) {
        java.lang.Integer a = 10;
        java.lang.Integer b = 10;
        java.lang.Integer c = 128;
        java.lang.Integer d = 128;
        int e = 128;
        //print true
        System.out.println(a==b);
        //print false
        System.out.println(c==d);
        //print ture
        System.out.println(c==e);
    }
}

a==b打印true,c==b打印false

a ==b 操作时jdk会做自动装箱操作，即调用Integer.valueof方法后通过==比较，valueOf会对-128 至127的数字做缓存

所以a==b比较的是同一个对像，而c,d是两个不同的对象==时返回false

c==e jvm对c做了拆箱操作 其实是c.intvalue==e它们比较的是值所有打印true

代码如下：

Integer.valueOf源码

/**
     * Returns an {@code Integer} instance representing the specified
     * {@code int} value.  If a new {@code Integer} instance is not
     * required, this method should generally be used in preference to
     * the constructor {@link #Integer(int)}, as this method is likely
     * to yield significantly better space and time performance by
     * caching frequently requested values.
     *
     * This method will always cache values in the range -128 to 127,
     * inclusive, and may cache other values outside of this range.
     *
     * @param  i an {@code int} value.
     * @return an {@code Integer} instance representing {@code i}.
     * @since  1.5
     */
    public static Integer valueOf(int i) {
        assert IntegerCache.high >= 127;
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

类加载器Integer时加载IntegerCache源码

private static class IntegerCache {
        static final int low = -128;
        static final int high;
        static final Integer cache[];

        static {
            // high value may be configured by property
            int h = 127;
            String integerCacheHighPropValue =
                sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
            if (integerCacheHighPropValue != null) {
                int i = parseInt(integerCacheHighPropValue);
                i = Math.max(i, 127);
                // Maximum array size is Integer.MAX_VALUE
                h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
            }
            high = h;

            cache = new Integer[(high - low) + 1];
            int j = low;
            for(int k = 0; k < cache.length; k++)
                cache[k] = new Integer(j++);
        }

        private IntegerCache() {}
    }

==与equals的区别

== 一般用于基本类型比较,==比较对象类型时比较的是引用

equals 用于比较对象类型，Integer,String之类的对像类型都有重写Object方法的equals方法

Integer.equals 源码

/**
     * Compares this object to the specified object.  The result is
     * {@code true} if and only if the argument is not
     * {@code null} and is an {@code Integer} object that
     * contains the same {@code int} value as this object.
     *
     * @param   obj   the object to compare with.
     * @return  {@code true} if the objects are the same;
     *          {@code false} otherwise.
     */
    public boolean equals(Object obj) {
        if (obj instanceof Integer) {
            return value == ((Integer)obj).intValue();
        }
        return false;
    }

总结：

基本类型 ==比较值是否相等

对像类型 ==比较是否同一对象，equals比较值是否相等

备注：源码来自JDK1.7