CODEWITHSUNDEEP
javastringswap
Chris
Chris

Reputation: 41

Swap String values without temporary variable

I am aware of overflow, performance drops, etc but I need to swap two String values without any temporary variable. I know what cons are and found some effective methods but cannot understand one of them.

String a = "abc";
String b = "def";

b = a + (a = b).substring(0, 0);

System.out.printf("A: %s, B: %s", a, b);

Output shows it clear values are swapped. When I look at this seems something related with priority operations but I cannot figure it out in my mind. Please, could someone explain to me what happens?

Upvotes: 4

Views: 294

Answers (4)

Vishnu Mari
Vishnu Mari

Reputation: 162

public class swapping { public static void main(String are[]) {

    //Swapping two Integer without temp variable 
    int a=10;
    int b=20;
    a=a+b;
    b=a-b;
    a=a-b;
    System.out.println(a+" "+b);
    
    //Swapping two string character with temp variable.
    String str1="Good";
    int as=str1.length();
    String str2="Morning";
    str1=str1+str2;
    str2=str1.substring(0, as);
    str1=str1.substring(as);
    System.out.println(str1+" "+str2);
}

}

Upvotes: 0

HGMamaci
HGMamaci

Reputation: 1387

Assuming both a and b are string variables:

a = (a+b).Substring((b=a).Length);

Upvotes: 0

Norbert Radyk
Norbert Radyk

Reputation: 2618

The fact you need to ask the question what's happening here, means you should seriously consider using constructions like this in your code.

Since in Java expressions are evaluated left to right (see here - 'Order of Evaluation' section).

This is what happens step-by-step in case of b = a + (a = b).substring(0, 0)

  • Populate a variable with its value:

    b = "abc" + (a = b).substring(0, 0);

  • Populate b variable with its value:

    b = "abc" + (a = "def").substring(0, 0);

  • Since first attribute of string concatenation (+) is already evaluated evaluate the second one, which starts with reassigning "def" to a.

    b = "abc" + "def".substring(0, 0); //Now a = "def"

  • Run substring on "def"

    b = "abc" + ""

  • Concatenate

    b = "abc"

  • Reassign to b and now b = "abc" (while as we've shown before a has been reassigned to "def".

Upvotes: 0

WoDoSc
WoDoSc

Reputation: 2618

Basically you can think on the swap operation

b = a + (a = b).substring(0, 0);

as

b = "abc" + (a = "def").substring(0, 0);

In this first step I simply substituted the variables with their values (except from the a in the parenthesis, since this is assigning a value, and not reading).

Now a is equal to "def" and therefore b is:

b = "abc" + "def".substring(0, 0);

In this step I substituted the parenthesis with its value. And now that is clearly:

b = "abc";

since the method .substring(0, 0) returns an empty string.

So the swap is done.

Upvotes: 7

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