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Expressions

An expression is any piece of code that evaluates down to a single value. Expressions are built by combining values, variables, and operators together. Every time you write 2 + 2, numMotors > 0, or velocity * 0.5, you're writing an expression.

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Expressions vs. Statements​

It's easy to mix up expressions and statements, but they aren't the same thing:

  • Expression: Evaluates to a value. Example: 5 + 3
  • Statement: A complete instruction that performs an action. Example: int sum = 5 + 3;

Every expression can be part of a statement, but not every statement is an expression. The assignment operator = takes the value produced by an expression on the right side and stores it in the variable on the left side.

Types of Expressions​

Arithmetic Expressions​

Combine numeric values using arithmetic operators (+, -, *, /, %).

int totalMotors = 4 + 2; // Evaluates to 6

Relational Expressions​

Compare two values and evaluate to a boolean (true or false).

boolean isFast = velocity > 5.0; // Evaluates to true or false

Logical Expressions​

Combine boolean values using logical operators (&&, ||, !).

boolean canShoot = hasNote && isAligned; // Evaluates to true or false

For more information about the specific operators used to build these expressions, see the Operators section.

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Evaluating Expressions​

When Java evaluates an expression made up of multiple operators, it doesn't just read left to right. Instead, it follows a strict set of precedence rules, similar to the order of operations you learned in math class (PEMDAS). For example:

int result = 2 + 3 * 4; // Evaluates to 14, not 20

Multiplication has a higher precedence than addition, so 3 * 4 is evaluated first, giving 12, and then 2 is added to get 14.

Using Parentheses​

Just like in math, parentheses can be used to override the default precedence and force part of an expression to be evaluated first.

int result = (2 + 3) * 4; // Evaluates to 20

It's good practice to use parentheses to make your intent clear, even when they aren't strictly necessary. This makes code easier for other people (and future you) to read.

Nested Expressions​

Expressions can be nested inside one another, since each expression simply evaluates down to a value that can then be used inside a larger expression.

int average = (numMotors + numSensors) / 2;

Here, numMotors + numSensors is evaluated first, producing a value that is then used as the left-hand side of the division expression.