一个有东西的需求：

要求在swap()方法里处理后输入a=2,b=1

    public static void main(String[] args) throws NoSuchFieldException {
        Integer a = 1;
        Integer b = 2;
        System.out.println("a = " + a + "\tb = " + b);
        swap(a, b);
        System.out.println("a = " + a + "\tb = " + b);
    }
    private static void swap(Integer a, Integer b) throws NoSuchFieldException {
      //TODO
    }

错误示例1

    private static void swap(Integer a, Integer b) throws NoSuchFieldException {
        Integer c = a;
        a = b;
        b = c;
    }

示例1原因解析

输出结果：
a = 1 b = 2
a = 1 b = 2
传递的是引用，一般来讲，是可以修改引用对应的对象的值，

示例2原因解析

执行查看java字节码问键命令

 javap -c IntegerExample.class

输出结果

Compiled from "IntegerExample.java"
public class net.wang.num.IntegerExample {
  public net.wang.num.IntegerExample();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return
       
  public static void main(java.lang.String[]) throws java.lang.NoSuchFieldException;
    Code:
       0: iconst_1
       1: invokestatic  #2                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
       4: astore_1
       5: iconst_2
       6: invokestatic  #2                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
       9: astore_2
      10: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
      13: new           #4                  // class java/lang/StringBuilder
      16: dup
      17: invokespecial #5                  // Method java/lang/StringBuilder."<init>":()V
      20: ldc           #6                  // String a =
      22: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      25: aload_1
      26: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
      29: ldc           #9                  // String          b =
      31: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      34: aload_2
      35: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
      38: invokevirtual #10                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      41: invokevirtual #11                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      44: aload_1
      45: aload_2
      46: invokestatic  #12                 // Method swap:(Ljava/lang/Integer;Ljava/lang/Integer;)V
      49: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
      52: new           #4                  // class java/lang/StringBuilder
      55: dup
      56: invokespecial #5                  // Method java/lang/StringBuilder."<init>":()V
      59: ldc           #6                  // String a =
      61: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      64: aload_1
      65: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
      68: ldc           #9                  // String          b =
      70: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      73: aload_2
      74: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
      77: invokevirtual #10                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      80: invokevirtual #11                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      83: return
}

这一行表示 Integer a = 1;实际上编译后变成Integer a = Integer.valueOf(1);

 1: invokestatic  #2                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;

Integer.valueOf(),属于自动装箱胡知识，源码如下：
在-128到127之间，直接从数组里取值

    //在-128到127之间，直接从数组里取值
    public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

IntegerCache的源码如下
初始化IntegerCache内部类，如果没有配置虚拟机参数：java.lang.Integer.IntegerCache.high，则取值-128~127，配置了，取值范围在-128~2^31次方之间

   private static class IntegerCache {
       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;
       }

       private IntegerCache() {}
   }