java-object-class

Every object in Java needs a basic set of operations — comparing with other objects, generating a hash code, producing a string representation, and being cloneable. Without a common base class, each class would redefine these fundamental behaviors inconsistently.

java.lang.Object is the root of the Java class hierarchy. Every class implicitly extends Object. It provides a set of core methods that all objects inherit: equals(), hashCode(), toString(), clone(), finalize(), getClass(), notify(), wait(), and notifyAll().

When a class is defined without an explicit extends clause, the compiler adds extends Object. All inherited methods can be overridden. The default equals() uses reference equality (==); hashCode() returns the memory address (typically); toString() returns ClassName@hashCode.

• equals/hashCode contract: If two objects are equal, they must have the same hash code
• Thread methods: wait() and notify() are defined here, making every object a monitor
• getClass(): Returns the runtime class — final, cannot be overridden
• finalize(): Called by GC before reclaiming memory (deprecated in Java 9+)

• Built from: Java Inheritance — Object is the ultimate parent of all classes
• Builds into: Java Strings — String overrides equals() and hashCode() for value comparison
• Builds into: Java Collections Framework — hashCode() is used by HashMap, HashSet
• Related: Java Polymorphism — Object reference can hold any type (polymorphism)

• equals() without hashCode(): Breaking the contract causes HashMap/HashSet to malfunction
• clone() is tricky: It performs a shallow copy; overriding requires implementing Cloneable
• finalize() is unreliable: Not guaranteed to run; use try-with-resources or Cleaner instead
• toString() default: ClassName@1a2b3c4d is usually not human-readable