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String EqualitySome surprises for string equality
Many programmers are sometimes suprised by Java's behavior re: string equality
Example 1Run the following code to see for yourself
public static void main(String[] args) {
String s0="a";
String s1 ="a";
String s2 ="b";
String s3 ="ab";
if((s1 + s2)==s3){
System.out.println("s1+s2 = s3");}
else{ System.out.println((s1+s2) +" does not equal " +s3);}
if(s1 == s2){
System.out.println(s1+" = " +s2);}
else if(s1 != s2)
System.out.println(s1+" != " +s2);
if(s1 == s0){
System.out.println("s1 = s0");}
else if(s1 != s0)
System.out.println("s1 != s0");
}
Example: concatating strings and string equality
public static void main(String[] args) {
String str1= "A";
String str2 ="B";
String str3 = "AB";
System.out.println("str1 = "+str1);
System.out.println("str2 = "+str2);
System.out.println("str3 = "+str3);
System.out.println("******assing str1+= str2 ******");
str1+= str2;
System.out.println(" now str1 = "+str1);
if(str3 == str1){System.out.println("str3 =str1");}
else{System.out.println("str3 !=str1");}
}
OUTPUT IS
str1 = A str2 = B str3 = AB ******assing str1+= str2 ****** now str1 = AB str3 !=str1The problem is that both strings, str1 & str3, refer to different string objects. And although both of these objects have the same letters, they are not the same object and therefore the comparison above does not work string.equals to the rescue!
The program below changes only one line!
public static void main(String[] args) {
String str1= "A";
String str2 ="B";
String str3 = "AB";
System.out.println("str1 = "+str1);
System.out.println("str2 = "+str2);
System.out.println("str3 = "+str3);
System.out.println("******assing str1+= str2 ******");
str1+= str2;
System.out.println(" now str1 = "+str1);
if(str3.equals(str1))
{System.out.println("str3 =str1");}
else{System.out.println("str3 !=str1");}
}
}
OUTPUT
str1 = A str2 = B str3 = AB ******assing str1+= str2 ****** now str1 = AB str3 =str1 |
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