1.需要自动拆箱装箱的类型
2. 基本类型及其包装类型
3.什么是自动拆箱装箱
装箱,就是将基本数据类型转换成包装器类型。
拆箱,就是自动将包装类型转换成基本数据类型
//自动装箱
Integer total = 99;
//自动拆箱
int totalprim = total;
看个栗子
public class StringTest {
public static void main(String[] args) {
//自动装箱
Integer total = 99;
//自定拆箱
int totalprim = total;
}
}
编译Java源码
javac StringTest.java
javap -c StringTest.class
javap 查看汇编命令结果
D:\ideaworkspace\test\src\testtool>javap -c StringTest.class
Compiled from "StringTest.java"
public class testtool.StringTest {
public testtool.StringTest();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: bipush 99
2: invokestatic #2 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
5: astore_1
6: aload_1
7: invokevirtual #3 // Method java/lang/Integer.intValue:()I
10: istore_2
11: return
}
自动装箱
执行 Integer total =99;
执行代码时系统为我们执行了 Integer total = Integer.valueOf(99);
这个就是自动装箱。
/**
* 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);
}
自动拆箱
执行 int totalprism = total 时 系统自动执行了
int totalprim = total.intValue();
/**
* Returns the value of this {@code Integer} as an
* {@code int}.
*/
public int intValue() {
return value;
}