• An enum may NOT be declared in a method
• An enum can be imported
• If the JVM has a choice, it will select a method without varargs before selecting a method with varargs
• When enums are equal, both .equals and == always return true
• The headMap() method returns the portion of the map whose keys are less than the key sent to it
• The asList() method of Arrays creates a fixed-size list that is backed by the array, so no additions are possible.
• A static initialization block is a normal block of code enclosed in braces, { }, and preceded by the static keyword. Used to initialize class variables
• All enums implicitly extend java.lang.Enum.
• A class can extend only one other class, an interface can extend any number of interfaces
• An interface might also contain constant definitions.
• An abstract class that implements an interface may not implement all interface methods

From: http://sitestree.com/?p=4872
Categories:Java Short Notes, SCJP
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Post Data:2012-12-07 07:48:57

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Interface

• An interface is a reference type, similar to a class
• Interfaces can contain only constants, method signatures, and nested types
• No method is implemented
• Interfaces cannot be instantiated
• They can only be implemented by classes or extended by other interfaces
• Interfaces are not part of the class hierarchy
• A class can implement multiple interfaces

From: http://sitestree.com/?p=4867
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Post Data:2010-01-20 15:33:28

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class declarations

• Start with modifiers such as public, private followed by class keyword
• The class name, with the initial letter capitalized
• The name of the class’s parent (superclass), preceded by the keyword extends (if any). A class can only extend (subclass) one parent.
• list of interfaces implemented by the class, preceded by the keyword implements (if any). A class can implement more than one interface
• The class body, surrounded by braces, {}.
• Class member variable declarations
  • Requires three components, in order:
  • Zero or more modifiers, such as public or private
  • The field’s type
  • The field’s name.

Abstract Classes

• A class declared with abstract keyword is an abstract class. It may or may not include abstract methods
• Abstract classes cannot be instantiated
• Abstract classes can be subclassed
• Class containing abstract methods, must be declared to be abstract
• Abstract methods are methods declared without implementation (braces)
• The subclass of an abstract class must provide implementations of all the abstract methods otherwise the subclass itself needs to be declared as abstract.
• An abstract class may have static and final fields and methods. Interfaces can not
• If an abstract class contains only abstract method declarations with no implementations, it should better be declared as an interface
• It’s a good design to declare a common abstract class with common methods with implementations for several very related classes (will extend the abstract class)
• An abstract class can implement an interface but are not bound to implement all interface methods

Nested Classes

• Classes declared under another class are called nested classes
• Two types: Static Nested: declared with static keyword, Inner Nested: declared with no static keyword
• inner classes have access to other members of the outer class including private members
• Static nested classes do not have access to other members of the outer class
• Static nested classes are accessed using the outer class name such as: OuterClass.StaticNestedClass
• To use inner classes, the outer class must be instantiated first. Then, inner object can be created as follows: OuterClass.InnerClass innerObject = outerObject.new InnerClass();
• Local inner classes: Declared within the body of a method
• Anonymous inner classes: Declared within the body of a method without naming it
• Inner classes may have similar access modifiers like other outer class members
• Why use nested classes:
  • For logically grouping classes that are only used in one place
  • It increases encapsulation.
  • May lead to more readable and maintainable code

From: http://sitestree.com/?p=4866
Categories:Java Short Notes, SCJP
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Post Data:2012-09-13 21:25:27

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Flow Control and ExceptionsFlow control statements

• Conditional: if, if-else and switch-case
• Looping: for, while, do-while
• Exception handling: try-catch-finally, throw
• Ad hoc flow control: break, continue with or without labels

switch statement
         switch(expression){ 
        case ConstantExpression: statement(s); 
        case ConstantExpression: statement(s);
        .
        .
        .
        default: statement(s);
      }

• expression: must be char, byte, short, or int, or a compile-time error occurs
• long primitive can be used if type casted to int
• Object reference cannot be used as expression
• Every case expression must be unique
• break statement may be used at the end of a case statement, to discontinue execution.
• There can be at most only one default statement.
• The order of case statements and default can be anything.

break and continue

• A break statement transfers the control out of an enclosing statement. break used within a loop breaks the execution of the current loop. In case of nested loops, the break statement passes the control to the immediate outer loop.
• A continue statement breaks the current iteration and moves to next iteration.
• break and continue with labels.
  • Labels specify the target (statement) for continue and break
  • continue with label does not jump to the labeled statement but instead jumps to the end of the labeled loop.
  • Same label identifiers can be reused multiple times as long as they are not nested.
  • Label names do not conflict with the same named identifier(variable, method or class name).

Checked and Unchecked Exceptions

Checked Exceptions

• Checked Exceptions are checked by the compiler to see if these exceptions are properly caught or specified. If not, the code will fail to compile
• Checked exception forces client program to deal with the scenario in which an exception may be thrown
• Checked exceptions must be either declared or caught at compile time

Unchecked Exceptions

• Unchecked exceptions are RuntimeException and all of its subclasses.
• Class java.lang.Error and its subclasses also are unchecked.
• Unchecked Exceptions are not checked by the compiler.
• Runtime exceptions do not need to be caught or declared.

From: http://sitestree.com/?p=4864
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Post Data:2008-08-25 00:56:33

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Garbage Collection

• Java itself does memory management. You do not need to allocate memory at the time of object creation; also you do not need to free memory explicitly
• Object is created either on the heap or on a stack
• Memory heap: Objects created with new keyword are placed in heaps. This memory remains allocated throughout the life cycle of the object. When the object is no more referred, the memory becomes eligible for garbage collection
• Stack: During method calls, objects are created for method arguments and method variables. These objects are created on stack. Such objects are eligible for garbage-collection when they go out of scope.
• Garbage Collection is a low-priority thread in java
• Garbage Collection cannot be forced explicitly. JVM may do garbage collection if it is running short of memory.
• The call System.gc() does NOT force the garbage collection but only suggests that the JVM may make an effort to do garbage collection.
• Garbage Collection is hardwired in Java runtime system. Java runtime system keeps the track of memory allocated. Therefore, it is able to determine if memory is still usable by any live thread. If not, then garbage collection thread will eventually release the memory back to the heap.
• Garbage Collection usually adopts an algorithm, which gives a fair balance between responsiveness (how quickly garbage-collection thread yields?) and speed of memory recovery (important for memory-intensive operations). Responsiveness is especially important in real time systems.
• An object is eligible for garbage collection when no object refers to it.
• An object also becomes eligible when its reference is set to null. (Actually all references to the object should be null for it to be eligible.)
• The objects referred by method variables or local variables are eligible for garbage collection when the method or their container block exits

From: http://sitestree.com/?p=4863
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Post Data:2009-03-10 13:59:45

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Class declaration and java source file.

• Only “one” top-level public class is allowed per java source file.
• The name of the java source file and the name of the top-level public class MUST be the same.
• If no public class is there in a file, after compiling separate class files will be created for all classes in the file.
• Package statement, import statements and class definition MUST appear in the order given.

Keywords and Identifiers

• Keywords are always in a lower case.
• Some keywords: const, goto, strictfp, and volatile
• Identifiers must start with either letter, $ or _ (underscore) and can have letter, $, _ or digits in it.
• no keyword is allowed as identifiers

Default Values, Local Variables

• Each primitive data type has a default value specified. Variable of primitive data type may be initialized
• Only class member variables are automatically initialized. Method variables need explicit initialization
• Local variables (also known as automatic variables) are declared in methods and in code blocks
• Automatic variables are not automatically initialized
• local variables should be explicitly initialized before first use. These are automatically destroyed when they go out of scope

Arrays

• Fixed-sized ordered collection of homogeneous data elements
  int[] ints; // array declaration
  ints = new ints[25]; // array construction at runtime.
• Array declared, constructed and initialized at the same time.
  int[] ints = {1,2,3,4,5}; // array declaration,
• An array of primitive data type created using the new keyword is automatically initialized. Each array element is initialized to its default value.
  For example,
  char[] arrayOfChars = new char[10];
• Array of object references: An array of object references created using the new keyword is also initialized. Each array element is initialized to its default value, i.e. null.
  String[] names = new String[10];
• length is a property of array (and not a method)
• java.lang.Object is the superclass of an array

Argument passing during method calls

• Always a copy of argument value is passed to calling method
• Arguments of primitive data types: first, a copy of the passing variable is made and then it is passed. The original copy remains unaffected
• Object reference as an argument: A copy of object reference is passed for method calls. It still points to the same object. The original copy is affected.

From: http://sitestree.com/?p=4862
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Post Data:2008-07-28 12:18:41

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Exams like SCJP test your understanding of Java operators and how to use them like:
assignment operators: =, +=, -=
arithmetic operators: +, -, *, /, %, ++, —
relational operators: < , , >=, ==, !=
logical operators: &, |, ^, !, &&, ||
conditional operators: ? :
Also operators to check the equality of two objects or two primitives

In the following table operators and their precedences are shown. Upper rows operators have higher precedence. Same row operators have equal precedence. For equal precedence operators, All binary operators except the assignment operators work from left to right while assignment operators work from right to left.

The following quick reference summarizes the operators supported by the Java programming language.

Simple Assignment Operator

=	Simple assignment operator

Arithmetic Operators

+ 	Additive operator (also used for String concatenation)- 	Subtraction operator*	Multiplication operator/ 	Division operator%	Remainder operator

Unary Operators

+ 	Unary plus operator; indicates positive value (numbers are positive without this, however)- 	Unary minus operator; negates an expression++  	Increment operator; increments a value by 1--    	Decrement operator; decrements a value by 1!     	Logical compliment operator; inverts the value of a boolean

Equality and Relational Operators

==	Equal to!=	Not equal to>	Greater than>=	Greater than or equal to< Less than<=	Less than or equal to

Conditional Operators

&& 	Conditional-AND|| 	Conditional-OR?:      Ternary (shorthand for if-then-else statement)

Type Comparison Operator

instanceof	Compares an object to a specified type 

Bitwise and Bit Shift Operators

~	Unary bitwise complement< >	Signed right shift>>>	Unsigned right shift&	Bitwise AND^	Bitwise exclusive OR|	Bitwise inclusive OR

Details about java operators

From: http://sitestree.com/?p=4848
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Post Data:2012-12-29 02:01:30

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