Class Hierarchies, Subclasses, and the Object Class

Put shared attributes and behavior in one parent class so every child class gets them for free. If Dog and Cat both need a name and an eat method, you write those once in Animal and let Dog and Cat extend Animal. This is inheritance: a subclass doesn't hold a pasted-in copy of the superclass's code, it holds a relationship. When you call a method on a Dog object, the JVM looks for that method starting at Dog, and if it isn't defined there, it walks up to Animal, then to Animal's superclass, and so on, running the first version it finds.

That walk always ends somewhere, because every class you write, directly or indirectly, extends Object. If a class has no explicit extends clause, Java silently makes it extend Object anyway. Object sits at the top of every hierarchy and defines a handful of methods every object inherits automatically, including toString and equals. So a chain like Circle extends Shape extends Object means a toString call on a Circle checks Circle first, then Shape, then Object, stopping at whichever level actually defines it.

This is exactly why any object can be dropped into a String with + concatenation: Java calls toString on it to get a String representation, and because toString traces back to Object, that call never fails to find a method, even if nobody along the chain bothered to override it. Understanding this lookup path matters once you start overriding methods yourself and calling super, since that keyword explicitly tells Java to look one level up rather than searching from the bottom.