When two threads are waiting each other and can’t precede the program is said to be deadlock.

In the context of Core Java, a deadlock refers to a situation where two or more threads are blocked forever, each waiting for the other to release a lock. Deadlocks can occur in multithreaded programs when there is a circular waiting dependency among two or more threads.

A deadlock typically involves the following four conditions, known as the Coffman conditions:

Mutual Exclusion: At least one resource must be held in a non-sharable mode (i.e., only one thread can use the resource at a time).
Hold and Wait: A thread must be holding at least one resource and waiting to acquire additional resources held by other threads.
No Preemption: Resources cannot be forcibly taken away from a thread holding them; they must be released voluntarily.
Circular Wait: There must be a circular chain of two or more threads, each waiting for a resource held by the next one in the chain.

When all these conditions are met, a deadlock can occur, leading to a situation where the involved threads are unable to make progress, resulting in a program that appears to be stuck.

Preventing deadlocks involves carefully managing the use of locks and resources to avoid the circular wait condition or ensuring that if it occurs, it is broken before reaching a deadlock state. Techniques such as lock ordering, deadlock detection, and deadlock avoidance are commonly used to handle and prevent deadlocks in multithreaded programs.