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Tips

an empty source file will compile without error

if a .java file does not contain a publicclass or interface it can have any name

a single-type import will take precedence over an import-on-demand import-on-demand types do not increase the size of the compiled code ie only the

types actually used are added to the code

while import-on-demand adds no overhead to the compiled code, they can slow downthe speed of the compile

a constructor body can include a return statement providing no value is returned

any method can throw a Runtime or Error exception without declaring it in the throwsclause

methods having the same name and parameter types do not have the same

signature unless the parameter types are listed in the same order

main() can be declared final

main() is inherited and can be overridden if not declared as final

args[0] references first command line argument after the application name ( arrays

in Java are zero-based)

main() can be declared public static void ... or static public void ...

the variable name does not have to be args; can be anything as long as

the type is String[]

variables can have the same name as a method or a class

only field variables are automatically initialized to their types default value; local

variables must be explicitly initialized

arrays are initialized to the default value of their type when they are created, not

declared, even if they are local variables

array index operator [] has highest level of precedence

integer variables can be used as array dimension values

postfix/prefix operators have the highest level of precedence

remember that when the postfix operator is used in an expression, the current valueof the variable is used

a class may be assigned to an Interface type if the class implements the interface orone of it's sub-interfaces

you cannot cast a primitive type to an object reference, or vice versa

you cannot cast a boolean type to another primitive type

String operations whose result does not alter the original string (ie calling

toUpperCase() on a String that is already in uppercase) return the original stringreference; otherwise they return a reference to a new String

Strings are immutable; the original String value can never be changed

all the primitive type wrapper classes override the Object.equals() method to

compare the value of the objects; the default Object.equals() method checks if the

variables reference the same object

you do not have to have a default statement in a switch() block

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the default statement in a switch() blcok can appear anywhere in the construct, does

not have to be last

all sections of the for() loop are optional

finalize() can only be executed once on any object

Traps

code with package or import declarations given in wrong order

more than one package declaration

file with more than one publicclass or interface declaration

filename.java does not match name ofpublicclass declared in the file

single-type imports for two classes in different packages but with the same simplename

single-type import with the same simple name as a class defined in the source file

attempting to import a package vs a type ie import java.util vsimport java.util.*

class attempting to extend more than one other class

class declared both final and abstract

an interface method declared as native or synchronized

an interface method declared as static

subclass with default constructor when the superclass does not have a no-argsconstructor or it's no-arg constructor has a throws clause

constructor declared with a return type

an abstract method also declared private, native, final, synchronized, or strictfp

an abstract method declared in a non-abstract class

a native or abstract method with a method body

method returning a type which is not convertible to the declared return type

a void method returning a value

a static method referencing this or super

main() declared other than according to the standard convention

local (automatic) variables declared with a modifier other than final

identifiers names beginning with a number or # sign

main listed as a possible keyword

capitalized words listed as possible keywords; particularly wrapper classes Integer,Boolean, etc

C/C++ keywords listed as possible Java keywords

an empty string vs null as the default value for a String object

incorrect array declaration statements, particularly:

arrayType [#] varName;

incorrect array initialization statements, particularly:arrayType[] varName = new arrayType[2];varName = { value, value, value };

negative values for array index

long value for array index

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array declaration used as an array creation statement

variables of primitive type handled as Objects

using the char literals \u000A or \u000D in comments or Strings

String literal "c" assigned to char type

using == operator to compare values of two different string reference variables

variables requiring narrowing conversion being passed to methods without usinga cast

assigning a typed byte or short variable to a char variable

floating point operation throwing an ArithmeticException

Bitwise operator precdence is: &^|

assigning subclasses with the same parent to each other

assigning a parent class to a subclass without a cast

result of an integer operation on byte or short types being assigned to a byte or short

without an explicit cast

a non-boolean value used for operand1 in a ternary expression

using == to compare the contents of two different String objects

using a new value based on a short-circuit operation that was never evaluated

code that results in a primitive value being changed in a method (can't happen)

code that results in an unchanged object value when it was changed in a method

failing to cast a value to match a method parameter type ie assuming narrowingconversion on a method call

a non-boolean value used in a loop or if( ) statement

using the assignment operator '=' vs '==' in an loop or if()statement

using an expression vs a value promotable to int in a switch()block

switch() blocks with duplicate case values

switch() blocks with incorrectly 'typed' case statements

switch() blocks with missing break statements (unintentionally causing code to fall

through to next case)

attempting to access a variable declared in the initialization outside of the for-loop

for()loop with incorrect initialization expression

for()loop with a non-boolean expression

a question that targets a specific object for garbage collection (can't be done)

a question that presumes to force the gc to run (can only suggest it run)