Abstract classes generally have at least one abstract method. However, this is not a requirement. Abstract classes may have a mix of both abstract and concrete methods, or they may have all concrete methods.

abstract void aProcedure();

For methods: There is no body associated with the method, only a semicolon marks the end of the method declaration.

Classes that contain abstract methods must themselves be abstract. However, abstract classes need not have any abstract methods.

boolean trueOrFalse;

Boolean variables can only be assigned the values true or false.

Always ends the innermost control structure in which the break statement is embedded. Iteration statements do not test to see if they should execute another cycle.

Labeled break statements end all nested control structures out to the iteration or switch statement with the same label.

byte tensOfThings;

Byte variables can be assigned 8-bit integer values between –128 and 127.

Within a switch statement, execution is suspended until a case value that matches the switch expression is found.

char aVowel;

Char variables can be assigned 16-bit Unicode characters.

The class is the basic building module of Java. It creates a generic description (or template, or blueprint) of one are more objects, the instances of the class. The class body lists the members of the class: the fields (or variables, or attributes), the methods (or functions), or local class/interface declarations.

Use final to indicate fields that are not changed after initialization.

The continue statement returns to the beginning of an embedded loop, at which time the loop tests itself whether to execute the body again or exit. The unlabeled continue goes to the beginning of the innermost nested loop. The labeled continue goes to the beginning of the iteration statement with that label.

If none of the case clauses in a switch statement match the switch expression, execution of statements within the switch block begins at the "default case" or "default label".

Executes a sequence of statements while the while condition is true. The iterated statements are executed at least once.

double astronomicalQuantity;

Double variables can be assigned 64-bit double-precision floating-point values. The largest possible double value is 1.79769313486231570e+308.

The else keyword is used only in conjunction with an if statement. The statement or block that follows the else is executed when the if test evaluates to false.

boolean allAnimalsAreDogs = false;

declares a class that can never be extended.

declares a method that can never be overridden in a subclass.

float aFloat;

Float variables can be assigned 32-bit single-precision IEEE floating point values. The largest possible float value is 3.40282347e+38f (note the "f").

Executes a statement (or block) repeatedly while a test condition is true.

Only break, continue, return and throw are permitted to disrupt structured flow.

If a class implements an interface, it guarantees to define all the methods specified in the interface. This is analogous to the class extending the interface and overriding its methods.

A class may implement more than one interface, in which case the interfaces are separated by commas in the class declaration:

Lets you access classes and interfaces that have already been created, especially the Java standard classes.

Determines whether the left-hand argument is an instance of a named class or interface or an instance of a subclass of the named class/interface. Evaluates to true or false.

int hundredsOfMillionsOfThings;

Int variables may be assigned 32-bit integer values. The largest possible int value is 2147483647, about two billion.

A specification of the methods which a class must in order to implement the interface.

long billionsOfBillionsOfThings;

Long variables may be assigned 64-bit integer values. The largest possible long value is 9223372036854775807L (note the "L" suffix), about nine billion, billion.

native void aProcedure();

Identifies methods that have bodies implemented outside of a Java class.

Creates a new instance of a class.

Creates an instance of an anonymous class that is assignment compatible with the named class/interface reference variables.

Allocates memory for an array.

There is no free, delete nor destructor mechanism within the Java language. Objects become subject to garbage collection when they are no longer referenced anywhere. All instances of a class do have a predefined (but not reserved) method, finalizer(), that is called prior to garbage collection, but there is no mechanism for forcing destruction.

MyClass variable=null;

Null is the reference value that does not indicate an object. The null value is assignment compatible with all reference variables.

package mypackage;

Declares the classes defined within a Java source file as parts of a named package. Packages are used by other source files through the import statement.

private int aField;

Private members may only be referenced within the class in which they are declared.

protected void aProcedure();

Protected members may be referenced within the class in which they are declared, classes in the same package, or subclasses.

Public classes and interfaces may be referenced outside the package in which they are declared.

Public members of public classes may be referenced outside the package in which the class is declared.

Exits from a void method. This statement is optional in a void method. If not present, the void method still exits when it reaches the end of the method body.

• return expression;

Exits a method that returns a value.

Short variables may be assigned 16 bit integer values.

Associates members with the class and not with the individual instances.

Static variables are 'shared' between all the instances of a class (as opposed to 'instance variables', for which each instance has an independant value).

strictfp double accountBalance;

Ensures that floating point values are normalized during calculations rather than just when assigned to floating point variables.

Invokes the constructor of the parent class from the constructor of a subclass. Must be the first executable statement within a constructor.

Invokes a method of the parent class that may have been overridden in a subclass.

switch( expression)

optional_default_case; // see default

Provides multiple entries into a sequence of statements. The switch expression (char or integer) is evaluated, and the statement following the case label that matches the value is executed. Subsequent statements are executed, even if they follow other case labels. The sequence of statements continues to be executed until break, continue or return is executed, or if an unhandled exception is thrown.

If no case matches the expression value, statements following the default label are executed.

• synchronized void aProcedure()

The synchronized methods of a class can only be executed by a single thread. Threads invoking a synchronized method in a class when another thread is executing a synchronized method will be suspended until the thread that acquired the "lock" on the class releases the lock.

• synchronized (reference)

A thread will wait until a lock is acquired on the reference before executing the statement. A lock can only be acquired for any given object by a single thread at a time.

aComponent.addListener(this);

A predefined reference variable defined for all non-static methods in a class. It provides a reference to the object whose method was called.

throw new Exception();

Interrupts the current sequence of statements being executed, and all parent structures and methods out to the innermost try statement.

void aProcedure() throws Exception

Identifies a procedure that either throws an exception directly, or does not handle the exception when it calls another method that throws the exception.

transient int temporary;

Ostensibly, transient identifies fields that are not "persistent" when the object is saved or "serialized". However, there is nothing in Java that takes advantage of this feature.

boolean allDogsAreAnimals = true;

Defines a section that could throw exceptions.

The statements in the try block are executed in sequence. If an exception occurs, and the exception class matches the type of the catch variable, then the statements in the catch block are executed, and the exception is deemed "handled". Regardless of whether or not an exception occurs, and whether it is handled or not, the statements in the finally block are executed.

Identifies a method that does not return a value.

volatile int threadFlag;

Identifies fields that must be reevaluated at every reference. Most programmers prefer just to use synchronized.