java IntegerCache

痛定思痛。 2022-05-17 10:21 167阅读 0赞

代码：

package cn.change;
    
    public class IntegerDemo {
    	public static void main(String[] args) {
    		Integer i1 = new Integer(127);
    		Integer i2 = new Integer(127);
                    //比较地址是否相同
    		System.out.println(i1==i2);//false
                    //比较值是否相同
    		System.out.println(i1.equals(i2));//true
    		System.out.println("-------------");
    		
    		Integer i3 = new Integer(128);
    		Integer i4 = new Integer(128);
    		System.out.println(i3==i4);//false
    		System.out.println(i3.equals(i4));//true
    		System.out.println("-------------");
    		
    		Integer i5 = 128;
    		Integer i6 = 128;
    		System.out.println(i5==i6);//false
    		System.out.println(i5.equals(i6));//true
    		System.out.println("-------------");
    		
    		Integer i7 = 127;
    		Integer i8 = 127;
    		System.out.println(i7==i8);//true
    		System.out.println(i7.equals(i8));//true
    	}
    }

输出结果：

false
    true
    -------------
    false
    true
    -------------
    false
    true
    -------------
    true
    true

分析：

integer源码：

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() {}
        }
    
        /**
         * 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) {
            if (i >= IntegerCache.low && i <= IntegerCache.high)
                return IntegerCache.cache[i + (-IntegerCache.low)];
            return new Integer(i);
        }

通过查看源码，我们可以发现，在public static Integer valueOf(int i);这个方法中，他会先判断i是否在-128到127之间，如果在，则直接从缓冲池中取相应的值，否则才new Integer(i);  所以在上面的比较中，i5 和 i6 的地址不同，但 i7 和 i8 的地址相同。