Declaration and Initialization of Variables in Java

Variable Declaration

• All variables must be declared before use.
• Necessary for the compiler to know the data type of the variable.
• Allows strict type checking.

Variable Initialization

• It is recommended to give the variable a value before using it.

Can be done by:

• Assignment Instruction.
• Initial value in the declaration.

int count = 10;  // valor inicial
char ch = 'X';   // inicializa com 'X'
float f = 1.2F;  // inicializa com 1.2

• You can initialize variables of the same type in a comma-separated list.

Dynamic Startup

• Variables can be dynamically initialized using valid expressions.

double volume = 3.1416 * radius * radius * height;

Scope and Lifetime of Variables

• Variables can be declared within any block.

A block defines a scope:

• Determines the visibility and lifetime of variables.
• Variables within a scope cannot be seen outside of it.

Scopes can be nested:

• Variables in external scopes are visible in internal scopes.
• Variables in internal scopes are not visible in external scopes.

int x = 10;
if(x == 10) {
  int y = 20;
  System.out.println("x and y: " + x + " " + y);
  x = y * 2;
}
System.out.println("x is " + x);

Scope Rules and Peculiarities

• Variables created when entering the scope and destroyed when leaving it.

• Variables are reset when re-entering the block.
  

for(int x = 0; x < 3; x++) {
  int y = -1;
  System.out.println("y is: " + y);
  y = 100;
  System.out.println("y is now: " + y);
}

• In Java, variables in internal scopes cannot have the same name as variables in external scopes.

// Este programa não será compilado
int count;
for(count = 0; count < 10; count++) {
  int count;  // inválido
  for(count = 0; count < 2; count++)
    System.out.println("This program is in error!");
}

Summary of Key Points

• Variables must be declared before use to enable type checking.
• Initialization can be done in declaration or dynamically.
• The scope of a variable determines where it can be accessed.
• Scopes can be nested and variables in the inner scope cannot have the same name as variables in the outer scope.
• The lifetime of a variable is confined to its scope.

The above is the detailed content of Declaration and Initialization of Variables in Java. For more information, please follow other related articles on the PHP Chinese website!