2302. Java Core - Tricks
Java

Some tricky cases when using Java.

1. Overflow when assigning Integer value

As we all know, the range of Integer in java is from -2147483648(-2^31) to 2147483647(2^31 - 1). And we use constant Integer.MIN_VALUE to represent -2147483648, and Integer.MAX_VALUE to represent 2147483647. When trying to increment the max value or decrement the min value, overflow occurs. That is

Integer.MAX_VALUE + 1 = Integer.MIN_VALUE;
Integer.MIN_VALUE - 1 = Integer.MAX_VALUE;

If you try to assign Integer.MAX_VALUE + 1 to a long type variable, the result may not be what you want.

long var1 = Integer.MAX_VALUE + 1; //var1 = -2147483648

You will get var1 = -2147483648 instead of desired 2147483648. The following statement doesn’t work, either.

long var1 = (long)(Integer.MAX_VALUE + 1);

The correct way is to cast the variable to long type before adding. The following two approaches will work.

long var1 = Integer.MAX_VALUE;
var1 = var1 + 1;

//or
long var1 = (long)Integer.MAX_VALUE + 1;

2. How a primitive float/double value can be -0.0?

Float is a tricky type in java.

2.1 The first issue is precision of float

You could never be able to store a floating point number of infinite precision with finite resources. You should never test if a floating point number == to some other, i.e. never write code like this:

if (a == b)

where a and b are floats. Due to rounding errors those two numbers might be stored as different values in memory. You should define a precision you want to work with:

private final static double EPSILON = 0.00001;

and then test against the precision you need

if (Math.abs(a - b) < epsilon)

2.2 The second is the negative zero value, -0.0.

When I working on an algorithm problem 149. Max Points on a Line in leetcode.com, I need to calculate the slope of two points, below are the original codes.

slope = (double)(points[i].y - points[j].y) / (points[i].x - points[j].x);

My program runs failed for one test case: [2,3],[3,3],[-5,3]. After debugging, I found the root cause is because of negative -0.0. The slope between point[2,3] and point[3,3] is -0.0. And the slope between point[2,3] and point[-5,3] is 0.0. But actually they are all in the same line with slope 0.0.

I updated my code to add 0.0 at the end of the line to make sure no negative zero appears. The problem is solved, and my solution passed all test cases.

slope = (double)(points[i].y - points[j].y) / (points[i].x - points[j].x) + 0.0;

2.3 Reference

3. Difference between i++ and ++i in a loop?

  • i++ => add 1 to a, returns the old value.
  • ++i => add 1 to a, returns the new value.
String[] items = {"a","b","c","d"};
int i = 0;
for (String item : items) {
    System.out.print(++i);
}
System.out.println();

i = 0;
for (String item : items) {
    System.out.print(i++);
}
System.out.println();

//output
1234
0123

The below two for loops print the same output.

for (int i = 0; i < 5; i++) {
    System.out.print(i);
}
System.out.println();
for (int i = 0; i < 5; ++i) {
    System.out.print(i);
}

//output
01234
01234

4. Is Java “pass-by-reference” or “pass-by-value”?

Everything in Java is pass-by-value. For class object, it pass its address to method.

public class Main{
    public static void main(String[] args){
        Foo f = new Foo("f");
        changeReference(f); // It won't change the reference!
        System.out.println(f.val); // Prints 'f'
        modifyReference(f); // It will modify the object that the reference variable "f" refers to!
        System.out.println(f.val); // Prints 'c'
        setToNull(f); // f wont't be null
        System.out.println(f.val); // Still prints 'c'
    }
    public static void changeReference(Foo a){
        Foo b = new Foo("b");
        a = b;
    }
    public static void modifyReference(Foo c){
        c.setAttribute("c");
    }
    public static void setToNull(Foo d){
        d = null;
    }
    private static class Foo {
        public String val = "";
        public Foo (String val) {
            this.val = val;
        }
        public void setAttribute(String val) {
            this.val = val;
        }
    }
}

Notice, method setToNull(Foo d) won’t work, object ‘f’ is still a non-null object.

4.2 Reference

Search the same code in the page of below link, there is an diagram explains why.

5. Convert Enum to String

When using Enum, we probably encounter the situation that we need to convert it to String. We can call ‘toString()’ method.

enum Color {
  RED, GREEN, BLUE;
}
public static void main(String[] args)
{
  Color c1 = Color.RED;
  String strColor = c1.toString();  // type cast
  System.out.println(strColor);
}

Better solution. Define the Enum type with String.

public enum Color {
   RED("RED"),
   GREEN("GREEN"),
   BLUE("BLUE");

   private String name;
   private Color (String name)
   {
       this.name = name;
   }
   public String getName()
   {
       return name;
   }
}

public static void main(String[] args)
{
  String strColor = Color.RED.getName(); // no type cast
  System.out.println(strColor);
}