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.

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.

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