2.2 - Boolean Expressions

You can compare values using relational operators:


== // equal to
!= // not equal to
<  // less than
>  // greater than
<= // less than or equal to
>= // greater than or equal to

With primitive types, == and != compare the actual values. With reference types (objects), they compare whether the two variables point to the same object in memory, not whether the objects look the same.


int a = 5;
int b = 5;
System.out.println(a == b); // true, comparing actual values

String s1 = new String("cat");
String s2 = new String("cat");
System.out.println(s1 == s2); // false, different objects even though content matches

To check whether two objects hold the same content (like two strings with the same characters), use the equals method instead. That’s covered in 2.6.

<, >, <=, and >= only work on numeric values, to compare their relationship.

You can compare an int with a double, like 3 < 3.5. You can’t use these operators on strings (that won’t compile). To put strings in alphabetical order, use compareTo from 1.15.


int x = 7;
int y = 10;
System.out.println(x < y);   // true
System.out.println(x > y);   // false
System.out.println(x <= 7);  // true
System.out.println(y >= 11); // false
System.out.println(x == y);  // false
System.out.println(x != y);  // true
System.out.println(3 < 3.5); // true, an int can be compared with a double

Any expression using these operators evaluates down to a boolean, either true or false.


boolean isAdult = age >= 18;

That boolean result can be stored in a variable (like isAdult), printed, or used directly as the condition of an if statement or a loop.

Don’t mix up = and ==. A single = stores a value (assignment), while == asks whether two values are equal. Writing if (x = 5) won’t compile, because x = 5 isn’t a boolean expression.

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