Java中String判断值为null或空及地址是否相等的问题

if (name == "") {
//do something
}

if (name.equals("")) {
//do something
}

if (！name.equals("")) {
//do something
}

if (name != null && ！name.equals("")) {
//do something
}

if (!"".equals(name)) {//将""写在前头，这样，不管name是否为null，都不会出错。
//do something
}

public class Test {
  public static void main (String args[]){
    String value = null;
    testNullOrEmpty(value);
 
    value = "";
    testNullOrEmpty(value);
 
    value = " ";
    testNullOrEmpty(value);
     
    value = "hello me";
    testNullOrEmpty(value);    
  }
     
  static void testNullOrEmpty(String value){
    if(value == null){
      System.out.println("value is null");
    } else if ("".equals(value)){
      System.out.println("value is blank but not null");
    } else {
      System.out.println("value is \"" + value + "\"");
    }
     
    if (value == "") { //NG 错误的写法 
      //别用这种写法 
    } 
  }
}

c:\>javac TestNullOrEmpty.java

c:\>java TestNullOrEmpty

value is null.
value is blank but not null.
value is " "
value is "hello me!"

package com; 
 
public class A 
{ 
 
  /** 
   * @param args 
   */ 
  public static void main(String[] args) 
  { 
 
    String a = "hello"; 
    String b = "he"; 
    String c = a.substring(0, 2); 
    System.out.println(b.equals(c));//true 
    System.out.println(b==c);//false 
     
    String d = new String("hello"); 
    System.out.println(d.equals(a));//true 
    System.out.println(d==a);//false 
     
    String e = new StringBuilder("hello").toString(); 
    System.out.println(e.equals(a));//true 
    System.out.println(e==a);//false 
     
    System.out.println(e.equals(d));//true 
    System.out.println(e==d);//false 
     
    String f = "hello"; 
    System.out.println(f.equals(a));//true 
    System.out.println(f==a);//true 
    System.out.println(f=="hello");//true 
    System.out.println(f=="hell"+"o");//true 
     
    String g = b+"llo"; 
    System.out.println(g==f);//false 
     
    String h = "he"+"llo"; 
    System.out.println(h==f);//true 
  } 
 
}