Java Musings - initializing a final variable with an uninitialized final variable

Java has fewer quirks compared to C++, but sometimes I do come across surprises. A code like following will fail to compile since you are trying to initialize a variable with an uninitialized variable.

Java

1
2
3
4
5
6
7
8
9
public class Sample {

  private final String mField1;
  private final String mField2 = mField1 + " two";

  private Sample(String field1) {
    mField1 = field1;
  }
}

But if instead of directly referencing mField1, you reference indirectly via a getter method code will compile, and mField2 will get “null” value for mField1.

Java

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
public class Sample {

  private final String mField1;
  private final String mField2 = getField1() + " two";

  private Sample(String field1) {
    mField1 = field1;
  }

  private String getField1() {
    return mField1;
  }

  private String getField2() {
    return mField2;
  }

  public static void main (String[] args) {
    Sample sample = new Sample("one");
    // prints "Field 1 is one"
    System.out.println("Field 1 is " + sample.getField1());
    // prints "Field 2 is null two"!!!
    System.out.println("Field 2 is " + sample.getField2());
  }
}

Related Posts