==运算符在Java中的对象

Question

我无法理解在以下代码中创建obj和obj2对象的方式的差异.特别是,我不确定如何将一个原语强制转换为一个对象.看看这里的其他一些问题,我认为这是不可能的.但是以下程序编译并运行正常.在第一种情况下,输出是假的,在第二种情况下,它是真的.

public class Test {

    public static void main(String args[]){

        Integer num = new Integer(3) ;
        Object obj = num;
        Integer[] integerArr = {1, 2, 3, 4};
        Object[] objArr = integerArr;
        boolean contains = false;

        for (int i = 0; i < objArr.length; i++){
            if (objArr[i] == obj){
                contains = true;
                break;
            }
        }

        System.out.println(contains);

        int num2 = 3 ;
        Object obj2 = num2;
        Integer[] integerArr2 = {1, 2, 3, 4};
        Object[] objArr2 = integerArr2;
        boolean contains2 = false;

        for (int i = 0; i < objArr2.length; i++){
            if (objArr2[i] == obj2){
                contains2 = true;
                break;
            }
        }

        System.out.println(contains2);
    }

}

Answer 1

你必须从以下角度理解这个方法java.lang.Integer

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);
}

在第一种情况下，您创建一个新Integer对象，而在第二种情况下，编译器通过使用缓存为您进行转换。

/**
 * Cache to support the object identity semantics of autoboxing for values between
 * -128 and 127 (inclusive) as required by JLS.
 *
 * The cache is initialized on first usage.  The size of the cache
 * may be controlled by the -XX:AutoBoxCacheMax=<size> option.
 * During VM initialization, java.lang.Integer.IntegerCache.high property
 * may be set and saved in the private system properties in the
 * sun.misc.VM class.
 */

这是相关的字节码，看看它最终如何调用Integer构造函数或Integer.valueOf：

0: new #2; //class java/lang/Integer
3: dup
4: iconst_3
5: invokespecial #3; //Method java/lang/Integer."<init>":(I)V
8: astore_1
9: aload_1
10: astore_2
11: iconst_4
12: anewarray #2; //class java/lang/Integer
15: dup
16: iconst_0
17: iconst_1
18: invokestatic #4; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
21: aastore
...
90: iconst_3
91: istore 6
93: iload 6
95: invokestatic #4; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
98: astore 7
100: iconst_4
101: anewarray #2; //class java/lang/Integer
104: dup
105: iconst_0
106: iconst_1
107: invokestatic #4; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
110: aastore
111: dup
...