Im getting a different result than the compiler when tracing array excercise

when i trace the code

You debug it. While debugging you figure several operations in one line:

for (int i =1; i<=4; i++)    
{
  a[i]=a[i-1]+i;
  System.out.print(a[i]+" ");
}

should be

for (int i =1; i<=4; i++)    
{
  int iNumber = a[i-1]; // get number of previous field
  iNumber += i; // add counter
  a[i]=iNumber; // place result
  System.out.print(a[i]+" ");
}

This way you can figure every step and see that the output if this is "1 3 6 10".