【作业】神奇的代码,包装类Integre,100==100,129!=129
1 public class StrangeIntegerBehavior 2 { 3 public static void main(String[] args) 4 { 5 Integer i1=100; 6 Integer j1=100; 7 System.out.println(i1==j1);//true 8 9 Integer i2=129; 10 Integer j2=129; 11 System.out.println(i2==j2);//false 12 13 } 14 }
问题:两对整数明明一模一样,为什么一个输出true一个输出false
安装jd-eclipse对该程序反编译如下
1 // Compiled from StrangeIntegerBehavior.java (version 10 : 54.0, super bit) 2 public class StrangeIntegerBehavior { 3 4 // Method descriptor #6 ()V 5 // Stack: 1, Locals: 1 6 public StrangeIntegerBehavior(); 7 0 aload_0 [this] 8 1 invokespecial java.lang.Object() [8] 9 4 return 10 Line numbers: 11 [pc: 0, line: 1] 12 Local variable table: 13 [pc: 0, pc: 5] local: this index: 0 type: StrangeIntegerBehavior 14 15 // Method descriptor #15 ([Ljava/lang/String;)V 16 // Stack: 3, Locals: 5 17 public static void main(java.lang.String[] args); 18 0 bipush 100 19 2 invokestatic java.lang.Integer.valueOf(int) : java.lang.Integer [16] 20 5 astore_1 [i1] 21 6 bipush 100 22 8 invokestatic java.lang.Integer.valueOf(int) : java.lang.Integer [16] 23 11 astore_2 [j1] 24 12 getstatic java.lang.System.out : java.io.PrintStream [22] 25 15 aload_1 [i1] 26 16 aload_2 [j1] 27 17 if_acmpne 24 28 20 iconst_1 29 21 goto 25 30 24 iconst_0 31 25 invokevirtual java.io.PrintStream.println(boolean) : void [28] 32 28 sipush 129 33 31 invokestatic java.lang.Integer.valueOf(int) : java.lang.Integer [16] 34 34 astore_3 [i2] 35 35 sipush 129 36 38 invokestatic java.lang.Integer.valueOf(int) : java.lang.Integer [16] 37 41 astore 4 [j2] 38 43 getstatic java.lang.System.out : java.io.PrintStream [22] 39 46 aload_3 [i2] 40 47 aload 4 [j2] 41 49 if_acmpne 56 42 52 iconst_1 43 53 goto 57 44 56 iconst_0 45 57 invokevirtual java.io.PrintStream.println(boolean) : void [28] 46 60 return 47 Line numbers: 48 [pc: 0, line: 5] 49 [pc: 6, line: 6] 50 [pc: 12, line: 7] 51 [pc: 28, line: 9] 52 [pc: 35, line: 10] 53 [pc: 43, line: 11] 54 [pc: 60, line: 13] 55 Local variable table: 56 [pc: 0, pc: 61] local: args index: 0 type: java.lang.String[] 57 [pc: 6, pc: 61] local: i1 index: 1 type: java.lang.Integer 58 [pc: 12, pc: 61] local: j1 index: 2 type: java.lang.Integer 59 [pc: 35, pc: 61] local: i2 index: 3 type: java.lang.Integer 60 [pc: 43, pc: 61] local: j2 index: 4 type: java.lang.Integer 61 Stack map table: number of frames 4 62 [pc: 24, full, stack: {java.io.PrintStream}, locals: {java.lang.String[], java.lang.Integer, java.lang.Integer}] 63 [pc: 25, full, stack: {java.io.PrintStream, int}, locals: {java.lang.String[], java.lang.Integer, java.lang.Integer}] 64 [pc: 56, full, stack: {java.io.PrintStream}, locals: {java.lang.String[], java.lang.Integer, java.lang.Integer, java.lang.Integer, java.lang.Integer}] 65 [pc: 57, full, stack: {java.io.PrintStream, int}, locals: {java.lang.String[], java.lang.Integer, java.lang.Integer, java.lang.Integer, java.lang.Integer}] 66 }
invokestatic java.lang.Integer.valueOf(int) : java.lang.Integer [16]
这里调用了静态类java.lang.Integer.valueOf(int),查看源码得
valueOf
public static Integer valueOf(int i)
Returns an
Integer
instance representing the specified int
value. If a new Integer
instance is not required, this method should generally be used in preference to the constructor 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.- Parameters:
i
- anint
value.- Returns:
- an
Integer
instance representingi
. - Since:
- 1.5
- 重点在于This method will always cache values in the range -128 to 127, inclusive, and may cache other values outside of this range.(该方法总是将值缓存在-128到127(包括在内)范围内,并且可能会缓存超出此范围的其他值。)
- i1和j1都是符合范围得数,而i2和j2大于128超出了范围
- 然后在还有一句话
- Returns an
Integer
object holding the value of the specifiedString
. The argument is interpreted as representing a signed decimal integer, exactly as if the argument were given to theparseInt(java.lang.String)
method. The result is anInteger
object that represents the integer value specified by the string.
In other words, this method returns an Integer
object equal to the value of:
new Integer(Integer.parseInt(s))
- 翻译:返回包含指定字符串值的整数对象。该参数被解释为表示一个带符号的十进制整数,就像该参数被赋给parseInt(java.lang.String)方法一样。结果是一个整数对象,表示字符串指定的整数值。换句话说,这个方法返回一个整数对象等于:新的整数(Integer.parseInt(s))
- 所以后面两个i2和j2不相等,因为他们是new出来的。
- 但这句话是写在java.lang.Integer.valueOf(String)里的,但是如果将代码改一下
1 public static void main(String[] args) 2 { 3 Integer i1=100; 4 Integer j1=100; 5 System.out.println(i1==j1);//true 6 7 Integer i2=12; 8 Integer j2=12; 9 System.out.println(i2==j2);//true 10 11 }
结果为:
true
true
就对了。。。