Threads in java

Problem 1:
You have thread T1, T2 and T3, how will you ensure that thread T2 start after the end of T1 and thread T3 will start after T2 end?
Solution:
By using Thread.join();
     t1.start(); 
     t1.join(); 
     t2.start(); 
     t2.join(); 
     t3.start(); 
//     t3.join();  - not needed

Problem 2:
"You have T1, T2, and T3. T1 and T2 can execute concurrently, but T3 has to wait for both of them before it can run."
Solution:
     t1.start(); 
     t2.start(); 
     t1.join(); 
     t2.join(); 
     t3.start(); 

Compare wait and sleep methods in java
Wait release the lock or monitor while sleep doesn't release any lock or monitor while waiting. Wait is used for inter-thread communication while sleep is used to introduce pause on execution.

Wait is defined in Object class, where sleep methods are defined in Thread class.

Both could throw InterruptedException.