Java will initialize fields with known default values so any explicit initialization of those same defaults is redundant and results in a larger class file (approximately three additional bytecode instructions per field). Example:
public class C {
  boolean b = false; // examples of redundant initializers
  byte by   = 0;
  short s   = 0;
  char c    = 0;
  int i     = 0;
  long l    = 0;

  float f   = .0f;   // all possible float literals
  doable d  = 0d;    // all possible double literals
  Object o  = null;

  MyClass mca[] = null;
  int i1 = 0, ia1[] = null;

  class Nested {
    boolean b = false;
  }
}