Statements

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Figure 130. Statements: General syntax Slide presentation

Statement's body terminated by “;”

{statement};

Figure 131. Statement examples: Declaring and assigning variables Slide presentation
Variable declaration:
int a;
Value assignment:
a = 33;
Combined declaration and assignment:
int a = 33;

Figure 132. Expression vs. statement Slide presentation
Expression:
a - 4
Statement:
b = a - 4;

Notice the trailing “;”.


Figure 133. Multiple statements per line Slide presentation 
a = b + 3; b = a - 4;

Discouraged by good coding practices:

  • Poor readability

  • Hampers debugging


Figure 134. Debugging multiple statements per line Slide presentation
Debugging multiple statements per line

Figure 135. Class scope Slide presentation

  • Variable being defined on class level.

  • Visible to all methods.

    public class X {
  // Class variable
  static int i = 3; ◀━━━━━━━━━━━━━━━━━━━━━━━━┓
                                              ┃
  public static void main(String[] args) {    ┃
    System.out.println("main: i=" + i);━━━━━━━┫
    someMethod();                             ┃
  }                                           ┃
  public static void someMethod() {           ┃
    System.out.println("someMethod: i=" + i);━┛
  }
}

Figure 136. Method local variable scope Slide presentation

  • Method scope being delimited by the { ... } block

  • A variable's visibility is restricted to its block:

    public class X {

  public static void main(String[] args) {
    int i = 1; // Visible in current method
    System.out.println("i=" + i);
    someMethod(); // Method call
  }

  public static void someMethod() {
    int j = 3;
    System.out.println("j=" + j);

    int i = 17; // No conflict with i in main(...)
    System.out.println("i=" + i);
  }
}

Figure 137. Nested blocks Slide presentation 
public static void main(String[] args) {

  double initialAmount = 34;

  { // first block
    final double interestRate = 1.2; // 1.2%
    System.out.println("Interest:" + initialAmount * interestRate / 100);
  }
  { // second block
    final double interestRate = 0.8; // 0.8%
    System.out.println("Interest:" + initialAmount * interestRate / 100);
  }
}

  • Defining scopes

  • Unit of work

  • if: Conditional block execution.

  • for / while: Repeated block execution.


exercise No. 55

Blocks and variable glitches

Q:

We consider:

public static void main(String[] args) {
   int a = 25;
      {
          int a = 30;
          System.out.println(a);
      }
}

This code does not compile due to an “Variable 'a' is already defined in the scope” error.

Why is that? Both definitions happen in different blocks.

A:

We do indeed have two blocks:

main() block:
public static void main(String[] args) {

    ...

}
Inner block:
{
   int a = 30;
   System.out.println(a);
}

However the inner block is nested inside the main block and thus inherits all variable declarations. It thus cannot redefine or shadow variable a from its enclosing parent.

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