can't java unchecked exceptions be handled using try/catch block?

The only difference between checked and unchecked exceptions is that checked ones have to be either caught or declared in the method signature using throws, whereas with unchecked ones this is optional.

Unchecked Exception can't be handled by your code i.e. we can't use try/catch block

Sure we can - but we don't have to.

Also i think throw keyword can be used only with try/catch block.can throw Keyword be used with Unchecked Exception?

Note that there are two keywords:

• throw explicitly throws an exception object you created. throw new NullPointerException(); works perfectly fine, though explicitly creating that particular exception is uncommon and most would consider it bad style.
• throws declares that a method may throw that exception. With unchecked exceptions this is optional, but can be useful to document the fact (again, one would normally not declared throws NullPointerException because that is pretty much a given).

All the unchecked exceptions can be treated in the same way as the checked ones - if you want, you can let them pass by declaring that the method throws them:

public void m() throws RuntimeException {}

Or you can catch them:

public void m() {
    try {
        // some code
    } catch (RuntimeException re) {
        // do something
    }
}

It should be noticed, the class RuntimeException acts as a catch-all for the unchecked exceptions (since all the unchecked exceptions extend from it), much in the same way that the Exception class is the catch-all for checked exceptions.

As has been mentioned before, the only real difference is that for checked exceptions you have to handle them (by letting them pass or catching them) and the compiler will make sure of it - on the other hand, the handling of unchecked exceptions is optional.

It all boils down to the expected usage of each exception type - you're supposed do be able to recover from checked exceptions (or at least do something about them, when they occur), whilst for unchecked exceptions, there might not be a reasonable way to recover from them. This of course, is a bit subjective.

They can be handled, but you don't have to. If you don't handle them, they will propagate and climb up the calling methods stack, until one of them catches it. If none does, the program will crash.

Usually, the bottom line is that if a client can reasonably be expected to recover from an exception, then it should be a checked exception. If a client cannot do anything to recover from the exception, then it's ok to have it as an unchecked exception.

Also, checked exceptions are useful to document an API that you expect to be used by 3rd-parties. When they know your method can throw a specific exception, they will code accordingly and handle the case. If you only use unchecked exceptions, all bets are off.

A common pattern (some people don't like it, but in some cases it's ok when you know what you're doing) is to wrap thrown checked exceptions into unchecked ones.

try {
   ... code that can throw CheckedException ...
} catch (CheckedException oopsSomethingBadHappened) {
    throw new RuntimeException("Something bad happened!", oopsSomethingBadHappened);
}