Non-final method invocation in constructor¶

ID: java/non-final-call-in-constructor
Kind: problem
Security severity: 
Severity: error
Precision: very-high
Tags:
   - quality
   - reliability
   - correctness
   - logic
Query suites:
   - java-code-quality.qls
   - java-security-and-quality.qls

Click to see the query in the CodeQL repository

If a constructor calls a method that is overridden in a subclass, it can cause the overriding method in the subclass to be called before the subclass has been initialized. This can lead to unexpected results.

Recommendation¶

Do not call a non-final method from within a constructor if that method could be overridden in a subclass.

Example¶

In the following example, executing new Sub("test") results in a NullPointerException. This is because the subclass constructor implicitly calls the superclass constructor, which in turn calls the overridden init method before the field s is initialized in the subclass constructor.

public class Super {
	public Super() {
		init();
	}
	
	public void init() {
	}
}

public class Sub extends Super {
	String s;
	int length;

	public Sub(String s) {
		this.s = s==null ? "" : s;
	}
	
	@Override
	public void init() {
		length = s.length();
	}
}

To avoid this problem:

The init method in the super constructor should be made final or private.
The initialization that is performed in the overridden init method in the subclass can be moved to the subclass constructor itself, or delegated to a separate final or private method that is called from within the subclass constructor.

References¶

J. Bloch, Effective Java (second edition), pp. 89–90. Addison-Wesley, 2008.
The Java Tutorials: Writing Final Classes and Methods.

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