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AKGBackup - The backup program

            

 

 

 

Points to remember

 

  • Everything from the start marker of a comment block (e.g. /*) is ignored until the first occurrence of the matching end marker (*/).

  • The "throw" statement can only throw "Throwable" objects. A NullPointerException will be thrown if the Exception given to the "throw" statement results in null.

  • In the same statement you can use decimal, octal and/or hexadecimal statements.

  • While creating a multidimensional array, dimensions must be created left to right. So, int [] [] array = new int [] [4]; is not valid.

  • Elements of an un-initialized array object get the default value corresponding to the type of the elements. Whether the array object reference is a local or member variable, does not matter.

  • In a switch statement the type of the case labels must be assignable to the type of switch expression.

  • It is not possible to break out of an "if" statement. If the "if" statement is placed within a labeled block, a switch statement or a loop construct, the usage of "break" would be valid.

  • The "if" statement does not return any value.

  • "continue" cannot be used outside the context of a loop.

  • A variable declared in a "try" block is not visible in "catch" or "finally" block.

  • If C extends B and B extends A, then it is not possible to call a method of A ( implemented in all the three classes ) from an instance method in class C.

  • An object that has been eligible for garbage collection may stop being eligible for garbage collection and return to normal life. This happens if the call to the finalize() reestablishes a reference from the active part of the program to the object.

  • If a class has a final blank variable, then it must be initialized when an instance is created ( in all the constructors or any initializer ) , or else the code will fail to compile.

  • A thread's status (user or daemon) can be changed before the thread is started. Changing the status of a thread already started throws IllegalStateException.

  • An exception which is not thrown in the "try" block (for example, thrown in a "catch" block) is not handled by subsequent "catch" blocks, but will rather be sent to the caller of the method. Before this happens, "finally" block is executed. Subsequent code after "finally" remains unreached if that exception remains uncaught after the execution of "finally" block.

  • If two or more interfaces declare a variable with the same name, then a class implementing these interfaces cannot refer to that variable by its simple name as it leads to ambiguity (even if the values of the variable is same in both the interfaces ). The ambiguity can be avoided by using fully qualified names e.g. Interface1.varName and Interface2.varName.

  • An object can be eligible for garbage collection even if there are references pointing to the object, as long as the objects with the references are themselves eligible for garbage collection.

  • The unary post-fix operators and all binary operators, except for assignment operators, associate from left to right. All other operators associate from right to left.

  • Floating point modulo (%) by zero produces NaN , but floating point divide by zero does not.

  • Integer division truncates to the next lower integer value in the absolute sense, but division by right shifting truncates to the next lower integer value in an algebraic sense. So, -7/2 == -3 but -7 >> 1 == -4.

  • Native methods cannot be "abstract", or "strictfp".

  • An abstract method cannot be static, final, synchronized, native, private, or strictfp.

  • After a thread has started, it will continue to run even if its reference is set to null. A running thread has a reference in a ThreadGroup even if your program does not retain one. Thus a running thread is never garbage collected.

  • A NullPointerException is thrown when an application attempts to use null in a case where an object is required.

  • A null reference may be used to access a static variable or method.

  • A local class cannot have any access modifier.

  • The "this" variable is not a normal reference variable that can be changed by statements like this = new A();   "this" cannot be used to refer to local variables.

 

 

  

 

 

 

 

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