Methods & Frames

public static void main(String[] args) 
{
  int i=1; 
  int a=4;
} 

• Frames are added to the stack at the highest available address (on the stack)
• Here we illustrate this by showing the stack with the highest address at the top
• The stack fills "downwards"

public static void main(String[] args) 
{
  int i=1; 
  int a=4;

  silly();
} 

void silly()
{
  float p=5.6f;
}

• When a method is called a new frame is added to the stack
• A method can only access its own frame
• Code in silly() cannot use the variables created in main()
• A frame limits the scope of a variable

public static void main(String[] args) 
{
  int i=1; 
  int a=4;

  silly();
} 

void silly()
{
  float p=5.6f;
  int i=3;
}

• This is why we can use the same variable name in different methods without changing the value everywhere

public static void main(String[] args) 
{
  int i=1; 
  int a=4;

  silly();
} 

void silly()
{
  float p=5.6f;
  int i=3;
  i--;
}

• This is why we can use the same variable name in different methods without changing the value everywhere

public static void main(String[] args) 
{
  int i=1; 

  i = silly(i);
} 

void silly(int locali)
{
}

• This is why we need to pass values as parameters if we want to use them in the method

public static void main(String[] args) 
{
  int i=1; 

  i = silly(i);
} 

void silly(int locali)
{
  locali = 3;
  return locali;
}

• Similarly if we want to pass back a value to the calling method we have to use a return