Is it OK to have if statement in the constructor

public class CTime {
  private int start_hour;
  private int end_hour;
  private int start_minute;
  private int end_minute;
  public CTime(int h1,int m1,int h2,int m2){
	  if(h1>=7 && h2<17 && h2*60+m2>h1*60+m1){

Yes. Just make sure that the new Object is has all its variables in a decent state regardless of which conditional paths are executed.

Constructor is a method.You can have if statement in a constructor

Thanks ,another question is there method witch is checking if the object arguments are different then null.

obj people(age,name,gender)

Do you mean like

if (age != null) ...

but you can only do this for reference types (objects), things like ints or floats can't be null anyway.

Thanks,I meant to check int!=0 && String!=null
in the same object

Yes, you can do exactly that.
The more interesting question is what will you do if you find invalid parameters in a constructor? How will the calling method know that its new Person(0, "Who?", 'X'); statement wasn't executed as it expected?
One option is to declare the constructor as throws (some kind of exception) and throw a new Exception if the params are invalid.

Thanks,could you give example with code how to do that

public Person(int age) throws Exception {
  if (age <= 0) throw new Exception("Age must be positive");
  this.age = age;