Classes

One step above methods is classes.

When classes have private fields and all interactions with those fields are mediated through methods

The ArrayList class that comes with Java looks something like this.¹

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    Object[] elementData;

    private int size;

    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    private void add(E e, Object[] elementData, int s) {
        if (s == elementData.length)
            elementData = grow();
        elementData[s] = e;
        size = s + 1;
    }

    private Object[] grow(int minCapacity) {
        int oldCapacity = elementData.length;
        if (oldCapacity > 0 || elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            int newCapacity = ArraysSupport.newLength(oldCapacity,
                    minCapacity - oldCapacity, /* minimum growth */
                    oldCapacity >> 1           /* preferred growth */);
            return elementData = Arrays.copyOf(elementData, newCapacity);
        } else {
            return elementData = new Object[Math.max(DEFAULT_CAPACITY, minCapacity)];
        }
    }

    private Object[] grow() {
        return grow(size + 1);
    }

    public boolean add(E e) {
        modCount++;
        add(e, elementData, size);
        return true;
    }

    // ...
}

People who write programs that depend on the calling .add on an ArrayList do not need to understand how or when the internal elementData and size fields are updated. Those also need not be the only fields that exist. All that is required is "calling .add will add an element to the list."

It is in this way that classes provide encapsulation. They can hide the fields which are needed and the way in which they are used - "implementation details" - in order to provide some implicit interface.