Operators in Java

Java's operators will look familiar if you've used any C-family language, but a couple of behaviors — especially around division and equality — surprise people coming from more forgiving languages.

Arithmetic operators

int sum = 5 + 3;        // 8
int difference = 5 - 3; // 2
int product = 5 * 3;    // 15
int quotient = 5 / 3;   // 1  -- integer division truncates!
int remainder = 5 % 3;  // 2

Integer division truncates the decimal part entirely — 5 / 3 is 1, not 1.666.... To get a decimal result, at least one operand needs to be a floating-point type:

double result = 5.0 / 3;  // 1.6666666666666667
double result2 = (double) 5 / 3; // same idea, cast one operand first

This is one of the most common early bugs in Java: dividing two ints and being confused why the fractional part vanished.

Increment, decrement, and compound assignment

int count = 0;
count++;        // count is now 1
count += 5;      // count is now 6
count -= 2;      // count is now 4

x++ (post-increment) returns the value before incrementing; ++x (pre-increment) increments first, then returns it. This matters when the expression is used immediately: int a = 5; int b = a++; leaves b as 5 and a as 6, while int b = ++a; would leave both at 6.

Comparison and logical operators

boolean isEqual = (5 == 5);       // true
boolean isGreater = (10 > 5);     // true
boolean bothTrue = (true && false); // false
boolean eitherTrue = (true || false); // true
boolean negated = !true;           // false

&& and || short-circuit: in a() && b(), if a() returns false, b() never runs at all, because the overall result is already determined. This is used deliberately all the time to guard against errors:

if (list != null && !list.isEmpty()) {
    // safe: if list is null, isEmpty() is never called
}

== vs. .equals()

For primitives, == compares values directly, exactly as expected. For objects — including String — == compares whether two references point to the same object in memory, not whether their contents are equal.

String a = new String("hi");
String b = new String("hi");
System.out.println(a == b);        // false -- different objects
System.out.println(a.equals(b));   // true -- same content

This trips up nearly every Java beginner at least once. The rule: use == for primitives, and .equals() to compare the content of objects, including strings.