ขอบเขตและอายุการใช้งาน

ขอบเขตกับช่วงอายุ

ขอบเขตของบล็อก

public class Main {
  public static void main(String[] args) {
    int a = 10; // visible in main
    {
      int b = 20; // visible only inside this block
      System.out.println("inside block: a=" + a + ", b=" + b);
    }
    System.out.println("outside block: a=" + a);
    // System.out.println(b); // would NOT compile: b is out of scope
  }
}

ขอบเขตของลูป

public class Main {
  public static void main(String[] args) {
    for (int i = 0; i < 3; i = i + 1) {
      System.out.println("i in loop = " + i);
    }
    // System.out.println(i); // would NOT compile: i is scoped to the loop

    int j;
    for (j = 0; j < 2; j = j + 1) {
      System.out.println("j in loop = " + j);
    }
    System.out.println("j after loop = " + j); // OK: j declared outside
  }
}

ฟิลด์กับตัวแปรภายใน

public class Main {
  // static field (class-level): lives for the program lifetime
  static int counter = 0;

  static void tick() {
    int local = 100; // local: created each call, dies when method returns
    counter = counter + 1;
    System.out.println("tick: counter=" + counter + ", local=" + local);
  }

  public static void main(String[] args) {
    tick();
    tick();
    tick();
    System.out.println("final counter=" + counter);
  }
}

การบดบังชื่อ

public class Main {
  static String label = "FIELD"; // class field

  static void demo(String label) { // parameter shadows field name
    System.out.println("param label = " + label); // uses parameter
    {
      String labelInner = "INNER"; // new name in inner block
      System.out.println("inner block labelInner = " + labelInner);
      System.out.println("still param label = " + label);
    }
    System.out.println("field via class name = " + Main.label); // refer to field explicitly
  }

  public static void main(String[] args) {
    demo("PARAM");
  }
}

พารามิเตอร์ค่าคงที่และการส่งค่าโดยสำเนา

public class Main {
  // final parameter cannot be reassigned inside the method
  static int inc(final int n) {
    // n = n + 1; // would NOT compile if uncommented
    return n + 1; // safe: return a new value
  }

  static void reassign(int x) {
    x = 999; // reassignes the local copy; caller variable is unchanged
  }

  public static void main(String[] args) {
    int a = 5;
    int b = inc(a);
    System.out.println("a=" + a + ", b=inc(a)=" + b);

    reassign(a);
    System.out.println("after reassign(a): a is still " + a);
  }
}

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