Lambda expressions and capturing variables

Access to Variables in the Contained Scope

• A lambda expression can access:
• Instance variables of the containing class.
• Static variables defined in the external class.
• The this parameter, which references the calling instance of the external class.

Access to Local Variables

• When using external scope local variables:
• The local variable must be effectively final.
• Effectively final: A variable whose value does not change after being assigned.
• Does not need to be explicitly declared as final.
• Modifying the local variable (inside or outside the lambda) invalidates its capture.

Modification Restriction
Cannot modify:

• Local variables captured within the lambda expression.
• Modifying these variables removes their effectively final status.

Modification of Instance Variables
A lambda expression can:

• Use and modify instance variables of the calling class.
• There is no restriction on "final" status for instance variables.

Example: Capturing Local Variable from External Scope
Code:

interface MyFunc {
    int func(int n);
}

class VarCapture {
    public static void main(String args[]) {
        // Variável local que pode ser capturada
        int num = 10;

        MyFunc myLambda = (n) -> {
            // Uso correto da variável num
            int v = num + n;

            // A instrução abaixo é inválida porque tenta modificar num
            // num++;

            return v;
        };

        // Usando a expressão lambda
        System.out.println(myLambda.func(8)); // Exibe: 18

        // Modificar num aqui também causaria erro
        // num = 9;
    }
}

Explanation:

• Num Capture:
  num is effectively final because it is not changed after the initial assignment.
  Lambda can access num, but cannot modify it.

• Execution:
  myLambda.func(8) returns 18 because:
  num (10) is added to the parameter n (8), producing 18.

• Error due to Modification:
  Uncommenting num or num = 9 would invalidate the capture, causing a compilation error.

• Note:
  Important: Instance or static variables can be used and modified without restrictions within lambda expressions.
  Restrictions apply only to external scope local variables.

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