Assignment and Compound Operators in Java

Assignment (=) stores a value in a variable. Java also has a full set of compound assignment operators (+=, -=, etc.) that combine an operation with assignment in one step. Assignments are expressions — they evaluate to the value assigned.

Basic assignment

int x = 10;
x = 20;                             // reassign
String s = "hello";

Compound assignment

The implicit cast

byte b = 1;
b = b + 1;           // ❌ compile error — result is int
b += 1;              // ✅ compound operator inserts an implicit cast

Compound operators silently narrow the result back to the left-hand type. Convenient, but occasionally surprising.

String +=

String s = "hello";
s += " world";                       // "hello world"
// Equivalent to s = new StringBuilder(s).append(" world").toString()

Fine for a few concatenations. Inside a loop, use a StringBuilder explicitly — repeated += allocates a new buffer every time.

Assignment returns a value

int y;
int z = (y = 10) + 5;               // z = 15, y = 10

String line;
while ((line = reader.readLine()) != null) {
    process(line);
}

Chained assignment

int a, b, c;
a = b = c = 0;                       // all three set to 0

Right-associative: the rightmost c = 0 evaluates to 0, then b = 0, then a = 0.

Common mistakes

• = where == was meant — if (flag = true) silently assigns.
• Chained assignment with different types — the compound form is simpler and clearer.
• Repeated += on strings in a loop — use StringBuilder.

Related

Pillar: Java operators. See also arithmetic, bitwise.