原因：
因为在Integer.class文件中，有一个静态内部类IntegerCache；
会在系统加载时，将（low = -128到h = 127）之间的数据提前包装成Integer对象放入数组cache中；
Integer a = 111;
Integer c = 111;
System.out.println(a==c);
valueOf()方法可以看出来，如果数值在（low = -128到h = 127）之间，会去IntegerCache.cache[]数组中查找并返回；
因为是同一个对象，所以使用 '= =' 判断肯定是true；如果不在（low = -128到h = 127），会创建一个新的Integer对象，用'= ='判断肯定就不等了；使用equals()对比，会将Integer先转为int值，再对比就相等了
注意：自动装箱使用的就是valueOf()实现的
@IntrinsicCandidate
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
public boolean equals(Object obj) {
if (obj instanceof Integer) {
return value == ((Integer)obj).intValue();
}
return false;
}
注意：自动拆箱使用的是intValue()实现的
@IntrinsicCandidate
public int intValue() {
return value;
}
举例：
自动拆箱：
int a = 1121;
Integer c = 1121;
System.out.println(a==c); // true 在比较时会将Integer的变量转为int类型比较，结果为true
public class JavaPTest {
public static void main(String[] args) {
// -128 127
int a = 1121;
Integer c = 1121;
System.out.println(a+c); //
}
}
自动装箱：
public class JavaPTest {
public static void main(String[] args) {
// -128 127
Integer a = Integer.valueOf("1121");
Integer c = 1121;
System.out.println(a == c); // false
}
}
同理
byte，short，int，long都是如此，区间默认都是（-128到127），Character是127
但是只有integer的最大范围是可以调整的；
配置java.lang.Integer.IntegerCache.high // 可以调整127的上限，不能大于int的最大值
源码:
IntegerCache源码：
private static class IntegerCache {
static final int low = -128;
static final int high;
static final Integer[] cache;
static Integer[] archivedCache;
static {
// high value may be configured by property
int h = 127;
String integerCacheHighPropValue =
VM.getSavedProperty("java.lang.Integer.IntegerCache.high"); // 可以调整127的上限，不能大于int的最大值
if (integerCacheHighPropValue != null) {
try {
h = Math.max(parseInt(integerCacheHighPropValue), 127);
// Maximum array size is Integer.MAX_VALUE
h = Math.min(h, Integer.MAX_VALUE - (-low) -1);
} catch( NumberFormatException nfe) {
// If the property cannot be parsed into an int, ignore it.
}
}
high = h;
// Load IntegerCache.archivedCache from archive, if possible
CDS.initializeFromArchive(IntegerCache.class);
int size = (high - low) + 1;
// Use the archived cache if it exists and is large enough
if (archivedCache == null || size > archivedCache.length) {
Integer[] c = new Integer[size];
int j = low;
for(int i = 0; i < c.length; i++) {
c[i] = new Integer(j++); // 包装成Integer对象放入数组
}
archivedCache = c;
}
cache = archivedCache;
// range [-128, 127] must be interned (JLS7 5.1.7)
assert IntegerCache.high >= 127;
}
private IntegerCache() {}
}
CharacterCache源码：
private static class CharacterCache {
private CharacterCache(){}
static final Character[] cache;
static Character[] archivedCache;
static {
int size = 127 + 1;
// Load and use the archived cache if it exists
CDS.initializeFromArchive(CharacterCache.class);
if (archivedCache == null || archivedCache.length != size) {
Character[] c = new Character[size];
for (int i = 0; i < size; i++) {
c[i] = new Character((char) i);
}
archivedCache = c;
}
cache = archivedCache;
}
}
LongCache源码：
private static class LongCache {
private LongCache() {}
static final Long[] cache;
static Long[] archivedCache;
static {
int size = -(-128) + 127 + 1;
// Load and use the archived cache if it exists
CDS.initializeFromArchive(LongCache.class);
if (archivedCache == null || archivedCache.length != size) {
Long[] c = new Long[size];
long value = -128;
for(int i = 0; i < size; i++) {
c[i] = new Long(value++);
}
archivedCache = c;
}
cache = archivedCache;
}
}
ByteCache源码：
private static class ByteCache {
private ByteCache() {}
static final Byte[] cache;
static Byte[] archivedCache;
static {
final int size = -(-128) + 127 + 1;
// Load and use the archived cache if it exists
CDS.initializeFromArchive(ByteCache.class);
if (archivedCache == null || archivedCache.length != size) {
Byte[] c = new Byte[size];
byte value = (byte)-128;
for(int i = 0; i < size; i++) {
c[i] = new Byte(value++);
}
archivedCache = c;
}
cache = archivedCache;
}
}
ShortCache源码
private static class ShortCache {
private ShortCache() {}
static final Short[] cache;
static Short[] archivedCache;
static {
int size = -(-128) + 127 + 1;
// Load and use the archived cache if it exists
CDS.initializeFromArchive(ShortCache.class);
if (archivedCache == null || archivedCache.length != size) {
Short[] c = new Short[size];
short value = -128;
for(int i = 0; i < size; i++) {
c[i] = new Short(value++);
}
archivedCache = c;
}
cache = archivedCache;
}
}
