String Concatenation Operator (`+`)

The + operator is one of the most interesting operators in Java because it serves two different purposes.

• It performs arithmetic addition when both operands are numeric.
• It performs String concatenation when at least one operand is a String.

This makes it the only overloaded operator in Java, making it a popular topic in Java interviews.

Let's understand how it works with simple examples.

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What Is the String Concatenation Operator?

Concatenation means joining two or more strings together.

Example

String firstName = "John";
String lastName = "Doe";

System.out.println(firstName + " " + lastName);

Output

John Doe

Here, the + operator joins multiple strings into a single string.

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The Only Overloaded Operator in Java

In Java, the + operator has two meanings.

Condition	Behavior
Both operands are numeric	Performs arithmetic addition
At least one operand is a String	Performs string concatenation

Examples

System.out.println(10 + 20);

Output

30

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System.out.println("Java" + " Programming");

Output

Java Programming

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System.out.println("Version " + 21);

Output

Version 21

Since one operand is a String, Java converts the other operand into a string and performs concatenation.

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Rule: Evaluation Happens from Left to Right

One of the most important rules is that the + operator is evaluated from left to right.

Once a String appears in the expression, every remaining + operation becomes string concatenation.

This is one of the most frequently asked Java interview concepts.

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Famous Interview Example

String a = "rajesh";
int b = 10;
int c = 20;
int d = 30;

System.out.println(a + b + c + d);
System.out.println(b + c + d + a);
System.out.println(b + c + a + d);
System.out.println(b + a + c + d);

Output

rajesh102030
60rajesh
30rajesh30
10rajesh2030

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Expression 1

a + b + c + d

Evaluation

"rajesh" + 10
↓

"rajesh10"

↓

"rajesh1020"

↓

"rajesh102030"

Output

rajesh102030

Since the first operand is a String, the remaining operations are concatenations.

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Expression 2

b + c + d + a

Evaluation

10 + 20
↓

30

↓

30 + 30

↓

60

↓

60 + "rajesh"

↓

"60rajesh"

Output

60rajesh

All numeric additions happen first because no String has appeared yet.

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Expression 3

b + c + a + d

Evaluation

10 + 20
↓

30

↓

30 + "rajesh"

↓

"30rajesh"

↓

"30rajesh30"

Output

30rajesh30

The first String changes all remaining operations into concatenation.

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Expression 4

b + a + c + d

Evaluation

10 + "rajesh"

↓

"10rajesh"

↓

"10rajesh20"

↓

"10rajesh2030"

Output

10rajesh2030

Since the second operand is a String, concatenation begins immediately.

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Assignment Examples

Given

String a = "rajesh";

int b = 10;
int c = 20;
int d = 30;

Statement	Result	Reason
a = b + c + d;	❌ Compile-time error	RHS is int, cannot assign to String
a = a + b + c;	✅ Valid	RHS is String
b = a + c + d;	❌ Compile-time error	RHS is String, cannot assign to int
b = b + c + d;	✅ Valid	RHS is int

Typical compile-time error

incompatible types
found: java.lang.String
required: int

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Command-Line Arguments Example

Command-line arguments are always received as strings.

Example

class Test {

    public static void main(String[] args) {

        System.out.println(args[0] + args[1]);

    }

}

Run

java Test 10 20

Output

1020

Many beginners expect the answer to be 30, but the output is 1020 because both command-line arguments are strings.

To perform arithmetic addition, convert them into integers.

int a = Integer.parseInt(args[0]);
int b = Integer.parseInt(args[1]);

System.out.println(a + b);

Output

30

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The += Operator Is Also Overloaded

The += operator can also perform string concatenation.

Example

String s = "rajesh";

s = s.concat("software");

System.out.println(s);

Output

rajeshsoftware

The same result can be achieved using the + operator.

String s = "rajesh";

s = s + "software";

System.out.println(s);

Or more simply,

String s = "rajesh";

s += "software";

System.out.println(s);

Output

rajeshsoftware

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Common Beginner Mistakes

Expecting Addition Instead of Concatenation

System.out.println("10" + 20);

Output

1020

Not

30

---

Forgetting Left-to-Right Evaluation

System.out.println(10 + 20 + "30");

Output

3030

Explanation

10 + 20 = 30

30 + "30"

↓

"3030"

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System.out.println("10" + 20 + 30);

Output

102030

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Best Practices

• Remember that the + operator is evaluated from left to right.
• If any operand is a String, the remaining operations become concatenation.
• Convert numeric strings using Integer.parseInt() when arithmetic is required.
• Use parentheses to make complex expressions easier to read.

Example

System.out.println("Total = " + (10 + 20));

Output

Total = 30

Without parentheses

System.out.println("Total = " + 10 + 20);

Output

Total = 1020

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Quick Memory Trick 🧠

Once a String enters the expression (left to right), every remaining + becomes concatenation.

Remember:

• Number + Number → Addition
• String + Anything → Concatenation

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Key Takeaways

• The + operator is the only overloaded operator in Java.
• It performs addition when both operands are numeric.
• It performs concatenation when at least one operand is a String.
• Java evaluates + expressions from left to right.
• Once a String appears, every remaining + performs concatenation.
• Command-line arguments are always strings.
• The += operator also supports string concatenation.
• Use parentheses whenever you want arithmetic to happen before concatenation.

Happy Coding!