Member Avatar for boyz

What is the need for calling default constructor by JVM? why the JVM should intiializes default values to the data fields if the constructor is not there in our class?

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Short answer: Java guarantees that every field has a well-defined default so objects never start with "garbage" memory. When an object is allocated the JVM zeroes its memory, so numeric fields get 0 (or 0.0), booleans false, char gets '\u0000', and reference fields get null. That zeroing happens before any constructor runs.

Clarification about the constructor: if you do not write any constructor the compiler generates a no-argument constructor for you (it basically contains a super() call). That generated constructor is not what supplies the default values — the defaults come from allocation/zero-initialization done by the VM. The compiler-generated constructor simply lets you call new MyClass() without an explicit constructor in source.

Practical initialization order (useful to remember): class static initialization runs first when the class is initialized; on new the object memory is allocated and zero-initialized; superclass constructors run; then the subclass’s instance variable initializers and initializer blocks run in textual order; finally the body of the constructor executes. A common gotcha: calling overridable methods from a superclass constructor can observe subclass fields still at their default values.

Tips: explicitly initialize fields in declarations or constructors when you need non-default invariants, and declare a no-arg constructor yourself if you also provide other constructors (the compiler won’t add one if you write any constructor). As suggested, the Java Language Specification gives the formal rules; the summary above covers the practical behavior you’ll see.

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Member Avatar for jwenting

learn English first. The word "doubt" is never used like that.
And with that newfound knowledge, read the language specification.

Member Avatar for Ancient Dragon

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