public class Test {

  public static void main(String[] args) {
    String a = "java書苑";
    String b = "java書苑";
    String c = new String("java書苑");
    String d = new String("java書苑").intern();

    if(a == b){
      System.out.println("a == b");
    }else{
      System.out.println("a != b");
    }

    if(a.equals(b)){
      System.out.println("a.equals(b)");
    }else{
      System.out.println("!a.equals(b)");
    }

    if(a == c){
      System.out.println("a == c");
    }else{
      System.out.println("a != c");
    }

    if(a.equals(c)){
      System.out.println("a.equals(c)");
    }else{
      System.out.println("!a.equals(c)");
    }

    if(a == d){
      System.out.println("a == d");
    }else{
      System.out.println("a != d");
    }

    if(a.equals(d)){
      System.out.println("a.equals(d)");
    }else{
      System.out.println("a.equals(d)");
    }
  }
}

a == b
a.equals(b)
a != c
a.equals(c)
a == d
a.equals(d)

public class Test {

  public static void main(String[] args) {

    int a = 127;
    int a1 = 127;

    int b = 128;
    int b1 = 128;


    Integer c = 127;
    Integer c1 = 127;

    Integer d = 128;
    Integer d1 = 128;

    if(a == a1){
      System.out.println("a == a1");
    }else{
       System.out.println("a != a1");
    }

    if(b == b1){
      System.out.println("b == b1");
    }else{
       System.out.println("b != b1");
    }

    if(c == c1){
      System.out.println("c == c1");
    }else{
       System.out.println("c != c1");
    }

    if(d == d1){
      System.out.println("d == d1");
    }else{
       System.out.println("d != d1");
    }
  }
}

a == a1
b == b1
c == c1
d != d1

/**
   * Cache to support the object identity semantics of autoboxing for values between
   * -128 and 127 (inclusive) as required by JLS.
   *
   * The cache is initialized on first usage. The size of the cache
   * may be controlled by the -XX:AutoBoxCacheMax=<size> option.
   * During VM initialization, java.lang.Integer.IntegerCache.high property
   * may be set and saved in the private system properties in the
   * sun.misc.VM class.
   */

  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) {
        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);
      }
      high = h;

      cache = new Integer[(high - low) + 1];
      int j = low;
      for(int k = 0; k < cache.length; k++)
        cache[k] = new Integer(j++);
    }

    private IntegerCache() {}
  }

  /**
   * 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);
  }