IOException vs RuntimeException Java

class Y {
    public static void main(String[] args) throws RuntimeException{//Line 1
        try {
            doSomething();
        }
        catch (RuntimeException e) {
            System.out.println(e);
        }
    }
    static void doSomething() throws RuntimeException{ //Line 2
        if (Math.random() > 0.5) throw new RuntimeException(); //Line 3
        throw new IOException();//Line 4
    }
}

When I throw two types of exception (IOException in Line4 and RunTimeException in Line3), I found that my program does not compile until I indicate "IOException" in my throws clauses in Line 1 & Line 2.

Whereas if I reverse "throws" to indicate IOException being thrown, the program does compile successfully as shown below.

class Y {
    public static void main(String[] args) throws IOException {//Line1
        try {
            doSomething();
        }
        catch (RuntimeException e) {
            System.out.println(e);
        }
    }
    static void doSomething() throws IOException {//Line 2
        if (Math.random() > 0.5) throw new RuntimeException();//Line 3
        throw new IOException();//Line 4
    }
}

Why should I always use "throws" for IOException even though RuntimeException is also thrown (Line 3) ?