Encapsulation vs abstraction

• Abstraction is about design: decide what an object should expose and ignore unnecessary detail. It answers "what does it do?" and is expressed with interfaces and abstract classes.
• Encapsulation is about implementation: bundle data with the methods that use it and hide the data behind access control, so it cannot be put into an invalid state. It answers "how do I protect how it does it?" and is expressed with private fields and methods.

public interface Thermostat { void setTarget(double celsius); }    // abstraction: the "what"

public class HomeThermostat implements Thermostat {
    private double target;                                          // encapsulation: hidden state
    public void setTarget(double celsius) {
        if (celsius < 5 || celsius > 35) throw new IllegalArgumentException("out of range");
        target = celsius;                                           // the class guards its invariant
    }
}

A good one-liner: abstraction hides complexity; encapsulation hides data (and protects invariants).