Java提供自动装箱拆箱的模式,下面例子说明其中的一些细节。
public class AutoBoxing {
public static void main(String[] args) {
Integer new3 = new Integer(3);
Integer int3toInteger = 3; // 将3自动装箱成Integer类型
Integer int3toInteger2 = 3; // 将3自动装箱成Integer类型
int int3 = 3;
System.out.println(new3 == int3toInteger);
// false 两个引用没有引用同一对象
System.out.println(int3toInteger == int3toInteger2);
// true 两个引用同在缓冲中
System.out.println(new3 == int3);
// true a自动拆箱成int类型再和c比较
Integer int129 = 129;
Integer int129_2 = 129;
System.out.println(int129 == int129_2);
// false -128~127是默认的int缓存范围
}
}
当我们给一个Integer对象赋一个int值的时候,会调用Integer类的静态方法valueOf,如果看看valueOf的源代码就知道发生了什么。
/**
* 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) {
assert IntegerCache.high >= 127;
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
问题显然。如果整型字面量的值在-128到127之间,那么不会new新的Integer对象,而是直接引用常量池中的Integer对象。
下面关注字符串的等等问题。
public static void main(String[] args) {
String s1 = "Programming";
String s2 = new String("Programming");
String s3 = "Program" + "ming";
System.out.println(s1 == s2);
System.out.println(s1 == s3);
System.out.println(s1 == s3.intern());
System.out.println(s2 == s2.intern());
}
输出:
false
true
true
false