Stack and Heap
❑ Local variables (method variables) live on the stack.
❑ Objects and their instance variables live on the heap.
Literals and Primitive Casting
❑ Integer literals can be decimal, octal (e.g. 013), or hexadecimal (e.g. 0x3d).
❑ Literals for longs end in L or l.
❑ Float literals end in F or f, double literals end in a digit or D or d.
❑ The boolean literals are true and false.
❑ Literals for chars are a single character inside single quotes: 'd'.
Scope
❑ Scope refers to the lifetime of a variable.
❑ There are four basic scopes:
❑ Static variables live basically as long as their class lives.
❑ Instance variables live as long as their object lives.
❑ Local variables live as long as their method is on the stack; however, if their method invokes another method, they are temporarily unavailable.
❑ Block variables (e.g., in a for or an if) live until the block completes.
Basic Assignments
❑ Literal integers are implicitly ints.
❑ Integer expressions always result in an int-sized result, never smaller.
❑ Floating-point numbers are implicitly doubles (64 bits).
❑ Narrowing a primitive truncates the high order bits.
❑ Compound assignments (e.g. +=), perform an automatic cast.
❑ A reference variable holds the bits that are used to refer to an object.
❑ Reference variables can refer to subclasses of the declared type but not to superclasses.
❑ When creating a new object, e.g., Button b = new Button();, three things happen:
❑ Make a reference variable named b, of type Button
❑ Create a new Button object
❑ Assign the Button object to the reference variable b
Using a Variable or Array Element That Is Uninitialized and Unassigned
❑ When an array of objects is instantiated, objects within the array are not instantiated automatically, but all the references get the default value of null.
❑ When an array of primitives is instantiated, elements get default values.
❑ Instance variables are always initialized with a default value.
❑ Local/automatic/method variables are never given a default value. If you attempt to use one before initializing it, you'll get a compiler error.
Passing Variables into Methods
❑ Methods can take primitives and/or object references as arguments.
❑ Method arguments are always copies.
❑ Method arguments are never actual objects (they can be references to objects).
❑ A primitive argument is an unattached copy of the original primitive.
❑ A reference argument is another copy of a reference to the original object.
❑ Shadowing occurs when two variables with different scopes share the same name. This leads to hard-to-find bugs, and hard-to-answer exam questions.
Array Declaration, Construction, and Initialization
❑ Arrays can hold primitives or objects, but the array itself is always an object.
❑ When you declare an array, the brackets can be left or right of the name.
❑ It is never legal to include the size of an array in the declaration.
❑ You must include the size of an array when you construct it (using new) unless you are creating an anonymous array.
❑ Elements in an array of objects are not automatically created, although primitive array elements are given default values.
❑ You'll get a NullPointerException if you try to use an array element in an object array, if that element does not refer to a real object.
❑ Arrays are indexed beginning with zero.
❑ An ArrayIndexOutOfBoundsException occurs if you use a bad index value.
❑ Arrays have a length variable whose value is the number of array elements.
❑ The last index you can access is always one less than the length of the array.
❑ Multidimensional arrays are just arrays of arrays.
❑ The dimensions in a multidimensional array can have different lengths.
❑ An array of primitives can accept any value that can be promoted implicitly to the array's declared type;. e.g., a byte variable can go in an int array.
❑ An array of objects can hold any object that passes the IS-A (or instanceof) test for the declared type of the array. For example, if Horse extends Animal, then a Horse object can go into an Animal array.
❑ If you assign an array to a previously declared array reference, the array you're assigning must be the same dimension as the reference you're assigning it to.
❑ You can assign an array of one type to a previously declared array reference of one of its supertypes. For example, a Honda array can be assigned to an array declared as type Car (assuming Honda extends Car).
Initialization Blocks
❑ Static initialization blocks run once, when the class is first loaded.
❑ Instance initialization blocks run every time a new instance is created. They run after all super-constructors and before the constructor's code has run.
❑ If multiple init blocks exist in a class, they follow the rules stated above, AND they run in the order in which they appear in the source file.
Using Wrappers
❑ The wrapper classes correlate to the primitive types.
❑ Wrappers have two main functions:
❑ To wrap primitives so that they can be handled like objects
❑ To provide utility methods for primitives (usually conversions)
❑ The three most important method families are
❑ xxxValue() Takes no arguments, returns a primitive
❑ parseXxx() Takes a String, returns a primitive, throws NFE
❑ valueOf() Takes a String, returns a wrapped object, throws NFE
❑ Wrapper constructors can take a String or a primitive, except for Character, which can only take a char.
❑ Radix refers to bases (typically) other than 10; octal is radix = 8, hex = 16.
Boxing
❑ As of Java 5, boxing allows you to convert primitives to wrappers or to convert wrappers to primitives automatically.
❑ Using == with wrappers created through boxing is tricky; those with the same small values (typically lower than 127), will be ==, larger values will not be ==.
Advanced Overloading
❑ Primitive widening uses the "smallest" method argument possible.
❑ Used individually, boxing and var-args are compatible with overloading.
❑ You CANNOT widen from one wrapper type to another. (IS-A fails.)
❑ You CANNOT widen and then box. (An int can't become a Long.)
❑ You can box and then widen. (An int can become an Object, via an Integer.)
❑ You can combine var-args with either widening or boxing.
Garbage Collection
❑ In Java, garbage collection (GC) provides automated memory management.
❑ The purpose of GC is to delete objects that can't be reached.
❑ Only the JVM decides when to run the GC, you can only suggest it.
❑ You can't know the GC algorithm for sure.
❑ Objects must be considered eligible before they can be garbage collected.
❑ An object is eligible when no live thread can reach it.
❑ To reach an object, you must have a live, reachable reference to that object.
❑ Java applications can run out of memory.
❑ Islands of objects can be GCed, even though they refer to each other.
❑ Request garbage collection with System.gc();
❑ Class Object has a finalize() method.
❑ The finalize() method is guaranteed to run once and only once before the garbage collector deletes an object.
❑ The garbage collector makes no guarantees, finalize() may never run.
❑ You can uneligibilize an object for GC from within finalize().
❑ Local variables (method variables) live on the stack.
❑ Objects and their instance variables live on the heap.
Literals and Primitive Casting
❑ Integer literals can be decimal, octal (e.g. 013), or hexadecimal (e.g. 0x3d).
❑ Literals for longs end in L or l.
❑ Float literals end in F or f, double literals end in a digit or D or d.
❑ The boolean literals are true and false.
❑ Literals for chars are a single character inside single quotes: 'd'.
Scope
❑ Scope refers to the lifetime of a variable.
❑ There are four basic scopes:
❑ Static variables live basically as long as their class lives.
❑ Instance variables live as long as their object lives.
❑ Local variables live as long as their method is on the stack; however, if their method invokes another method, they are temporarily unavailable.
❑ Block variables (e.g., in a for or an if) live until the block completes.
Basic Assignments
❑ Literal integers are implicitly ints.
❑ Integer expressions always result in an int-sized result, never smaller.
❑ Floating-point numbers are implicitly doubles (64 bits).
❑ Narrowing a primitive truncates the high order bits.
❑ Compound assignments (e.g. +=), perform an automatic cast.
❑ A reference variable holds the bits that are used to refer to an object.
❑ Reference variables can refer to subclasses of the declared type but not to superclasses.
❑ When creating a new object, e.g., Button b = new Button();, three things happen:
❑ Make a reference variable named b, of type Button
❑ Create a new Button object
❑ Assign the Button object to the reference variable b
Using a Variable or Array Element That Is Uninitialized and Unassigned
❑ When an array of objects is instantiated, objects within the array are not instantiated automatically, but all the references get the default value of null.
❑ When an array of primitives is instantiated, elements get default values.
❑ Instance variables are always initialized with a default value.
❑ Local/automatic/method variables are never given a default value. If you attempt to use one before initializing it, you'll get a compiler error.
Passing Variables into Methods
❑ Methods can take primitives and/or object references as arguments.
❑ Method arguments are always copies.
❑ Method arguments are never actual objects (they can be references to objects).
❑ A primitive argument is an unattached copy of the original primitive.
❑ A reference argument is another copy of a reference to the original object.
❑ Shadowing occurs when two variables with different scopes share the same name. This leads to hard-to-find bugs, and hard-to-answer exam questions.
Array Declaration, Construction, and Initialization
❑ Arrays can hold primitives or objects, but the array itself is always an object.
❑ When you declare an array, the brackets can be left or right of the name.
❑ It is never legal to include the size of an array in the declaration.
❑ You must include the size of an array when you construct it (using new) unless you are creating an anonymous array.
❑ Elements in an array of objects are not automatically created, although primitive array elements are given default values.
❑ You'll get a NullPointerException if you try to use an array element in an object array, if that element does not refer to a real object.
❑ Arrays are indexed beginning with zero.
❑ An ArrayIndexOutOfBoundsException occurs if you use a bad index value.
❑ Arrays have a length variable whose value is the number of array elements.
❑ The last index you can access is always one less than the length of the array.
❑ Multidimensional arrays are just arrays of arrays.
❑ The dimensions in a multidimensional array can have different lengths.
❑ An array of primitives can accept any value that can be promoted implicitly to the array's declared type;. e.g., a byte variable can go in an int array.
❑ An array of objects can hold any object that passes the IS-A (or instanceof) test for the declared type of the array. For example, if Horse extends Animal, then a Horse object can go into an Animal array.
❑ If you assign an array to a previously declared array reference, the array you're assigning must be the same dimension as the reference you're assigning it to.
❑ You can assign an array of one type to a previously declared array reference of one of its supertypes. For example, a Honda array can be assigned to an array declared as type Car (assuming Honda extends Car).
Initialization Blocks
❑ Static initialization blocks run once, when the class is first loaded.
❑ Instance initialization blocks run every time a new instance is created. They run after all super-constructors and before the constructor's code has run.
❑ If multiple init blocks exist in a class, they follow the rules stated above, AND they run in the order in which they appear in the source file.
Using Wrappers
❑ The wrapper classes correlate to the primitive types.
❑ Wrappers have two main functions:
❑ To wrap primitives so that they can be handled like objects
❑ To provide utility methods for primitives (usually conversions)
❑ The three most important method families are
❑ xxxValue() Takes no arguments, returns a primitive
❑ parseXxx() Takes a String, returns a primitive, throws NFE
❑ valueOf() Takes a String, returns a wrapped object, throws NFE
❑ Wrapper constructors can take a String or a primitive, except for Character, which can only take a char.
❑ Radix refers to bases (typically) other than 10; octal is radix = 8, hex = 16.
Boxing
❑ As of Java 5, boxing allows you to convert primitives to wrappers or to convert wrappers to primitives automatically.
❑ Using == with wrappers created through boxing is tricky; those with the same small values (typically lower than 127), will be ==, larger values will not be ==.
Advanced Overloading
❑ Primitive widening uses the "smallest" method argument possible.
❑ Used individually, boxing and var-args are compatible with overloading.
❑ You CANNOT widen from one wrapper type to another. (IS-A fails.)
❑ You CANNOT widen and then box. (An int can't become a Long.)
❑ You can box and then widen. (An int can become an Object, via an Integer.)
❑ You can combine var-args with either widening or boxing.
Garbage Collection
❑ In Java, garbage collection (GC) provides automated memory management.
❑ The purpose of GC is to delete objects that can't be reached.
❑ Only the JVM decides when to run the GC, you can only suggest it.
❑ You can't know the GC algorithm for sure.
❑ Objects must be considered eligible before they can be garbage collected.
❑ An object is eligible when no live thread can reach it.
❑ To reach an object, you must have a live, reachable reference to that object.
❑ Java applications can run out of memory.
❑ Islands of objects can be GCed, even though they refer to each other.
❑ Request garbage collection with System.gc();
❑ Class Object has a finalize() method.
❑ The finalize() method is guaranteed to run once and only once before the garbage collector deletes an object.
❑ The garbage collector makes no guarantees, finalize() may never run.
❑ You can uneligibilize an object for GC from within finalize().
No comments:
Post a Comment