Nesting

When you put one control statement, such as an if, inside another control statement, you have created nested control statements.

Worded another way, a nested control statement is one that is under the control of another.

if ( c < 10 )
  if ( resp == 'y' )  // nested
    System.out.println("Today is a warm day");

if ( c < 10 )
  System.out.println("Today is a warm day");
else
  if ( resp == 'n' )  // nested
    System.out.println("Today is a cold day");

This would be more readable with curly braces {} - look at the next set of statements.

if ( x < y )
  if ( a == b )
    System.out.println("abc");
else
  System.out.println("sxyz"); // Whose "else" IS this?

* Indentation is irrelevant to program logic (but critical to your code's readability): the else here is paired to the closest if that it follows, the a == b condition.

Using code blocks makes this clearer and prevents bugs.

if ( x < y ) {
  if ( a == b ) {
    System.out.println("abc");
  }
}
else {
  System.out.println("sxyz"); // Whose "else" IS this?
}

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