• if super or this are called, they can only be called on the first line of the constructor
• an exception can be thrown if a parameter is invalid
• you should ensure that the constructed object is in a valid state
• constructors should never call an overridable method (an overridable method is one which is neither private, static, nor final)
• constructors are never synchronized
• constructors can create thread objects, but the thread should not be started within a constructor
• constructors shouldn't pass a this reference to other objects
• constructors can be private, in order to restrict construction

• the default constructor is the constructor provided by the system in the absence of any constructor provided by the programmer. Once a programmer supplies any constructor whatsoever, the default constructor is no longer supplied.
• a no-argument constructor, on the other hand, is a constructor provided by the programmer which takes no arguments.

Example

Resto is an immutable class, since its state cannot change after construction. Note that:

• all validation is performed in its constructor
• almost all of its javadoc is concerned with stating precisely what is to be passed to the constructor

import java.math.BigDecimal;
import java.util.Objects;

/** Model Object for a Restaurant. */
public final class Resto {

  /**
   Full constructor.
    
   @param id underlying database internal identifier (optional) 1..50 characters
   @param name of the restaurant (required), 2..50 characters
   @param location street address of the restaurant (optional), 2..50 characters
   @param average price of a meal (optional) $0.00..$100.00
   @param comment on the restaurant in general (optional) 2..50 characters
  */
  public Resto(
    String id, String name, String location, BigDecimal price, String comment
  ) throws ModelCtorException {
    this.id = id;
    this.name = Objects.requireNonNull(name);
    this.location = location;
    this.price = price;
    this.comment = comment;
    validateState();
  }
  
  public String getId() { return id; }
  public String getName() {  return name; }
  public String getLocation() {  return location;  }
  public BigDecimal getPrice() { return price; }
  public String getComment() {  return comment; }

  /** For debugging only. */
  @Override public String toString(){
    return id + ":" + name;
  }
  
  @Override public boolean equals(Object aThat) {
    if (this == aThat) return true;
    if (!(aThat instanceof Resto)) return false;
    Resto that = (Resto)aThat;
    for(int i = 0; i < this.getSigFields().length; ++i){
      if (!Objects.equals(this.getSigFields()[i], that.getSigFields()[i])){
        return false;
      }
    }
    return true;
  }  
  
  @Override public int hashCode(){
    return Objects.hash(getSigFields());
  }
  
  // PRIVATE 
  private final String id;
  private final String name;
  private final String location;
  private final BigDecimal price;
  private final String comment;
  
  /** Does all of the checks specified in the constructor's javadoc.*/
  private void validateState() throws ModelCtorException {
    //...elided
  }
  
  private Object[] getSigFields(){
    return new Object[] {name, location, price, comment};
  }
}