In the Java programming language, threads are a fundamental part of concurrent programming. They enable multiple operations to run simultaneously, improving the performance of applications. Understanding how to properly manage threads is crucial in Java programming, especially when it comes to operations like initializing and starting threads.

One of the questions that often arises is whether it is legal to call the `start()` method twice on the same `Thread` object. In this article, we'll dive deep into this topic to understand the legalities, the consequences, and the best practices surrounding this subject.

The `Thread` Class in Java

Java's `Thread` class provides a platform to create and control threads. To execute a thread in Java, you typically instantiate a `Thread` object, configure it, and then call its `start()` method. The `start()` method is responsible for beginning the thread's execution; once it's called, the thread moves to the 'Runnable' state.

Creating a thread can be done by either:

1. Extending the `Thread` class.
2. Implementing the `Runnable` interface.

The general steps are:

• Once a thread has been started, its state changes from 'New' to 'Runnable'.
• A thread can only transition from 'New' to 'Runnable' once—after that, its state transitions between 'Runnable', 'Blocked', 'Waiting', and 'Terminated'.
• Allowing a re-start would contradict the lifecycle of threads as defined in Java, complicating the management of thread states.
• Avoid Double Starting: As seen, calling `start()` twice is illegal. Use proper checks to ensure a thread is in the correct state before invoking `start()`.
• State Management: Employ a thread-safe mechanism to track the state of a thread if necessary. Libraries like `java.util.concurrent` offer utilities that help manage threads safely.
• Resource Management: Threads are resources; improper management can lead to resource leaks. Use frameworks or patterns (like the Executor framework) that provide thread management facilities.
• Creating a New Thread: If you need to re-run logic within a thread, simply instantiate a new `Thread` object each time.
• Thread Pools: For repetitive tasks, use thread pools such as those provided by the `ExecutorService`. This approach is more resource-efficient and handles threads under the hood.