Java 自动拆箱装箱原理

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;
 }

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