先看一个例子
public class CompareTo {
public static void main(String[] args) {
Long a1 = 128L;
Long a2 = 128L;
System.out.println("a1==a2 : " + (a1==a2));
System.out.println("a1==128L : " + (a1==128L));
Long a3 = 127L;
Long a4 = 127L;
System.out.println("a3==a4 :" + (a3==a4));
System.out.println("a3==127L : " + (a3==127L));
}
}
输出结果：
a1==a2 : false
a1==128L : true
a3==a4 : true
a3==127L : true
查看Long类中的源码，方法中的返回值的方式
/**
* Returns a {@code Long} instance representing the specified
* {@code long} value.
* If a new {@code Long} instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Long(long)}, as this method is likely to yield
* significantly better space and time performance by caching
* frequently requested values.
*
* Note that unlike the {@linkplain Integer#valueOf(int)
* corresponding method} in the {@code Integer} class, this method
* is <em>not</em> required to cache values within a particular
* range.
*
* @param l a long value.
* @return a {@code Long} instance representing {@code l}.
* @since 1.5
*/
public static Long valueOf(long l) {
final int offset = 128;
if (l >= -128 && l <= 127) { // will cache
return LongCache.cache[(int)l + offset];
}
return new Long(l);
}
而Integer也符合这种情况，查看源代码
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;
}
/**
* 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);
}
然后发现Long和Integer都对值进行了范围控制，如果超出范围就执行 new 一个新的对象，否则就直接执行 cache属性，而cache的操作是：
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;
}
其他的类型，与这两个的操作不太一样，而Short 则是在范围内就强制转换成short，否则就抛出NumberFormatException异常
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