string s1 = null;
string s2 = null;
system.out.println(s1==s2);//true
//system.out.println(s1.equals(s2));//nullpointerexception
s1 = s2;
system.out.println(s1==s2);//true
//system.out.println(s1.equals(s2));//nullpointerexception
system.out.println("***1***");
s1 = null;
s2 = "";
system.out.println(s1==s2);//false
//system.out.println(s1.equals(s2));//nullpointerexception
s1 = s2;
system.out.println(s1==s2);//true
system.out.println(s1.equals(s2));//true
system.out.println("***2***");
s1 = "";
s2 = null;
system.out.println(s1==s2);//false
system.out.println(s1.equals(s2));//false
s1 = s2;
system.out.println(s1==s2);//true
//system.out.println(s1.equals(s2));//nullpointerexception
system.out.println("***3***");
s1 = "";
s2 = "";
system.out.println(s1==s2);//true
system.out.println(s1.equals(s2));//true
s1 = s2;
system.out.println(s1==s2);//true
system.out.println(s1.equals(s2));//true
system.out.println("***4***");
s1 = new string("");
s2 = new string("");
system.out.println(s1==s2);//false
system.out.println(s1.equals(s2));//true
s1 = s2;
system.out.println(s1==s2);//true
system.out.println(s1.equals(s2));//true
system.out.println("***5***");
s1 = "null";
s2 = "null";
system.out.println(s1==s2);//ture
system.out.println(s1.equals(s2));//true
s1 = s2;
system.out.println(s1==s2);//true
system.out.println(s1.equals(s2));//true
system.out.println("***6***");
s1 = new string("null");
s2 = new string("null");
system.out.println(s1==s2);//flase
system.out.println(s1.equals(s2));//true
s1 = s2;
system.out.println(s1==s2);//true
system.out.println(s1.equals(s2));//true
system.out.println("***7***");
s1 = "abc";
s2 = "abc";
system.out.println(s1==s2);//ture
system.out.println(s1.equals(s2));//true
s1 = s2;
system.out.println(s1==s2);//true
system.out.println(s1.equals(s2));//true
system.out.println("***8***");
s1 = new string("abc");
s2 = new string("abc");
system.out.println(s1==s2);//false
system.out.println(s1.equals(s2));//true
s1 = s2;
system.out.println(s1==s2);//true
system.out.println(s1.equals(s2));//true
system.out.println("***9***");
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