Integer類型數據的比較

我們在做Integer數據比較的時候，會有意想不到的結果，通過看Integer的源碼，問題迎刃而解。這裏的論述建立在jdk版本是1.8

Intger類有個靜態內部類IntegerCache，是用於緩存Intger對象的。這裏會緩存-128 到127的數據。

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) {
                try {
                    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);
                } catch( NumberFormatException nfe) {
                    // If the property cannot be parsed into an int, ignore it.
                }
            }
            high = h;

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

            // range [-128, 127] must be interned (JLS7 5.1.7)
            assert IntegerCache.high >= 127;
        }

        private IntegerCache() {}
    }

我們再看Integer.valueOf(int i)方法，如果請求的對象是-128-127之間，則直接返回緩存中的對象，如果不在這個區間，則會新建一個對象返回。

 public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

我們運行下栗子：

package com.leadbank.util;

public class IntegerTest {

    /**
     * Integer,初始化的時候會在緩存裏存入-128-127的數據，只要
     * 通過Integer.valueOf(x)獲取數據在這個返給內，都會從緩存中獲取
     * @param args
     */
    public static void main(String[] args) {
        Integer a = new Integer(4);//只要自己new了，就會自己開闢內存空間
        Integer b =4;//會被編譯器解析爲Integer b = Integer.valueOf(4);
        System.out.println("new Integer(4)=Integer 4:"+(a==b));
        int c = 4;//會被解析爲a.intValue() == c
        System.out.println("new Integer(4)=int 4:"+(a==c));
        Integer d = Integer.valueOf(4);
        //false,因爲a開闢了自己的內存空間，沒有從緩存中獲取
        System.out.println("new Integer(4)=Integer.valueOf(4):"+(a==d));
        Integer e = 4;
        System.out.println("Integer 4 = Integer 4:"+(b==e));//true

        //結果true
        Integer _nagative_first_equal_128 = -128;
        Integer _nagative_second_equal_128 = -128;
        System.out.println("_nagative_first_equal_128=_nagative_second_equal_128:"+(_nagative_first_equal_128==_nagative_second_equal_128));

        //結果false
        Integer _nagative_first_equal_129 = -129;
        Integer _nagative_second_equal_129 = -129;
        System.out.println("_nagative_first_equal_129=_nagative_second_equal_129:"+(_nagative_first_equal_129==_nagative_second_equal_129));


        //結果true
        Integer _first_equal_127 = 127;
        Integer _second_equal_127 = 127;
        System.out.println("_first_equal_127=_second_equal_127:"+(_first_equal_127==_second_equal_127));

        //結果false
        Integer _first_equal_128 = 128;
        Integer _second_equal_128 = 128;
        System.out.println("_first_equal_128=_second_equal_128:"+(_first_equal_128==_second_equal_128));
    }
}