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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.dataflow.cfg.node;

import com.sun.source.tree.ArrayAccessTree;
import com.sun.source.tree.ExpressionTree;
import com.sun.source.tree.Tree;

import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.dataflow.qual.SideEffectFree;
import org.checkerframework.javacutil.TreeUtils;

import java.util.Arrays;
import java.util.Collection;
import java.util.Objects;

/**
 * A node for an array access:
 *
 *  *   arrayref [ index ]
 * 
 *
 * We allow array accesses without corresponding AST {@link Tree}s.
 */
public class ArrayAccessNode extends Node {

    /** The corresponding ArrayAccessTree. */
    protected final ArrayAccessTree tree;

    /** The array expression being accessed. */
    protected final Node array;

    /** The index expresssion used to access the array. */
    protected final Node index;

    /**
     * If this ArrayAccessNode is a node for an array desugared from an enhanced for loop, then the
     * {@code arrayExpression} field is the expression in the for loop, e.g., {@code arr} in {@code
     * for(Object o: arr}.
     *
     * Is set by {@link #setArrayExpression}.
     */
    protected @Nullable ExpressionTree arrayExpression;

    /**
     * Create an ArrayAccessNode.
     *
     * @param t tree for the array access
     * @param array the node for the array expression being accessed
     * @param index the node for the index used to access the array
     */
    public ArrayAccessNode(ArrayAccessTree t, Node array, Node index) {
        super(TreeUtils.typeOf(t));
        this.tree = t;
        this.array = array;
        this.index = index;
    }

    /**
     * If this ArrayAccessNode is a node for an array desugared from an enhanced for loop, then
     * return the expression in the for loop, e.g., {@code arr} in {@code for(Object o: arr}.
     * Otherwise, return null.
     *
     * @return the array expression, or null if this is not an array desugared from an enhanced for
     *     loop
     */
    public @Nullable ExpressionTree getArrayExpression() {
        return arrayExpression;
    }

    /**
     * Set the array expression from a for loop.
     *
     * @param arrayExpression array expression
     * @see #getArrayExpression()
     */
    public void setArrayExpression(@Nullable ExpressionTree arrayExpression) {
        this.arrayExpression = arrayExpression;
    }

    /**
     * Get the node that represents the array expression being accessed.
     *
     * @return the array expression node
     */
    public Node getArray() {
        return array;
    }

    public Node getIndex() {
        return index;
    }

    @Override
    public ArrayAccessTree getTree() {
        return tree;
    }

    @Override
    public  R accept(NodeVisitor visitor, P p) {
        return visitor.visitArrayAccess(this, p);
    }

    @Override
    public String toString() {
        String base = getArray().toString() + "[" + getIndex() + "]";
        return base;
    }

    @Override
    public boolean equals(@Nullable Object obj) {
        if (!(obj instanceof ArrayAccessNode)) {
            return false;
        }
        ArrayAccessNode other = (ArrayAccessNode) obj;
        return getArray().equals(other.getArray()) && getIndex().equals(other.getIndex());
    }

    @Override
    public int hashCode() {
        return Objects.hash(getArray(), getIndex());
    }

    @Override
    @SideEffectFree
    public Collection getOperands() {
        return Arrays.asList(getArray(), getIndex());
    }
}