这个问题刚遇见的时候很是奇怪，先贴上测试代码
public class SOlution { public static void main(String[] args){ Integer a=1000; Integer b=1000; System.out.println(a==b); Integer a1=100; Integer b1=100; System.out.println(a1==b1); } }
测试结果：
false;
true;
很是傻眼，不信你去试试；理论上来说两个Integer类用“==”比较的话怎么也应该是false,怎么到了100就变成true了呢？
原因解析：需要查看java中Integer中的源代码（见下方）。下面源代码中标红的代码段是重点。Integer类中有个内部类：IntegerCache ，这个类中有一个缓存数组cache[]，存放的内容是-128到127的整数（看蓝色代码）。最后再看Integer类中的valueOf()方法，在-128到127之间的数之间返回cache数组中的Integer，否则新建一个Integer类，本质就在这，这就解释了为什么a1和b1两个integer引用地址是一样的了，因为他们根本就是同一个实例。而a和b是两个不同的Integer实例
JAVA Integer类源码
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) { 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); }
