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

e

• 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;

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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);

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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);
}

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• 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!");
}

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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.

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