Java Expressions, Statements and Blocks

In previous chapters, we have used expressions, statements, and blocks without much explaining about them. Now that you know about variables, operators, and literals, it will be easier to understand these concepts.

1. Java Expressions

A Java expression consists of variables, operators, literals, and method calls. It evaluates to a single value.

int score;
score = 90;

Here, score = 90 is an expression that returns an int.

Examples:

• a + b
• number1 == number2
• "Hello " + "World"

2. Java Statements

In Java, each statement is a complete unit of execution. It's like a complete sentence in the English language.

int score = 9 * 5;

Here, 9 * 5 is an expression, but int score = 9 * 5; is a statement.

Types of Statements

1. Expression Statements: Expressions terminated by a semicolon.

   number = 10;
   ++number;
   System.out.println("Hello");

2. Declaration Statements: Used to declare variables.

   double tax = 9.5;

3. Control Flow Statements: Used to control the flow of execution (if, while, for).

3. Java Blocks

A block is a group of statements (zero or more) that is enclosed in curly braces { }.

class Main {
    public static void main(String[] args) {
        String band = "Beatles";

        if (band == "Beatles") { // start of block
            System.out.print("Hey ");
            System.out.print("Jude!");
        } // end of block
    }
}

Key Points:

• Blocks can be nested inside other blocks.
• Variables declared inside a block are only visible within that block (Scope).

Example: Nested Blocks

public class Main {
    public static void main(String[] args) {
        // Outer block
        int x = 10;
        {
            // Inner block
            int y = 20;
            System.out.println(x + y); // 30
        }
        // System.out.println(y); // Error! y is not visible here
    }
}

Key Takeaways

• Expression: Evaluates to a value (e.g., 5 + 3).
• Statement: A complete unit of execution, usually ending with ;.
• Block: A group of statements enclosed in { }.

Challenge