Java syntax
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| Modifier | Extended Class inside another package |
|---|---|
| private | no |
| default (package private) | no |
| protected | yes |
| public | yes |
| Modifier | Non-extended inside another package |
|---|---|
| private | no |
| default (package private) | no |
| protected | no |
| public | yes |
Constructors and initializers
A constructor is a special method called when an object is initialized. Its purpose is to initialize the members of the object. The main differences between constructors and ordinary methods are that constructors are called only when an instance of the class is created and never return anything. Constructors are declared as common methods, but they are named after the class and no return type is specified:
class Foo {
String str;
Foo() { // Constructor with no arguments
// Initialization
}
Foo(String str) { // Constructor with one argument
this.str = str;
}
}
Initializers are blocks of code that are executed when a class or an instance of a class is created. There are two kinds of initializers, static initializers and instance initializers.
Static initializers initialize static fields when the class is created. They are declared using the static keyword:
class Foo {
static {
// Initialization
}
}
A class is created only once. Therefore, static initializers are not called more than once. On the contrary, instance initializers are automatically called before the call to a constructor every time an instance of the class is created. Unlike constructors instance initializers cannot take any arguments and generally they cannot throw any checked exceptions (except in several special cases). Instance initializers are declared in a block without any keywords:
class Foo {
{
// Initialization
}
}
Since Java has a garbage collection mechanism, there are no destructors. However, every object has a finalize() method called prior to garbage collection, which can be overridden to implement finalization.
Methods
All the statements in Java must reside within methods. Methods are similar to functions except they belong to classes. A method has a return value, a name and usually some parameters initialized when it is called with some arguments. Similar to C++, methods returning nothing have return type declared as void. Unlike in C++, methods in Java are not allowed to have default argument values and methods are usually overloaded instead.
class Foo {
int bar(int a, int b) {
return (a*2) + b;
}
/* Overloaded method with the same name but different set of arguments */
int bar(int a) {
return a*2;
}
}
A method is called using . notation on an object, or in the case of a static method, also on the name of a class.
Foo foo = new Foo();
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