• numbers: 0 or 0.0
• booleans: false
• object references: null

This means that explicitly setting fields to 0, false, or null (as the case may be) is unnecessary and redundant. Since this language feature was included in order to, in part, reduce repetitive coding, it's a good idea to take full advantage of it. Insisting that fields should be explicitly initialized to 0, false, or null is an idiom which is likely inappropriate to the Java programming language.

Furthermore, setting a field explicitly to 0, false, or null may even cause the same operation to be performed twice (depending on your compiler).

Example

public final class Quark {

  public Quark(String aName, double aMass){
    fName = aName;
    fMass = aMass;
  }

  //PRIVATE

  //WITHOUT redundant initialization to default values
  //private String fName;
  //private double fMass;

  //WITH redundant initialization to default values
  private String fName = null;
  private double fMass = 0.0d;
}