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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.block;

import org.checkerframework.checker.initialization.qual.UnknownInitialization;
import org.plumelib.util.ArraySet;

import java.util.Set;
import java.util.concurrent.atomic.AtomicLong;

/** Base class of the {@link Block} implementation hierarchy. */
public abstract class BlockImpl implements Block {

    /** The type of this basic block. */
    protected final BlockType type;

    /** The set of predecessors. */
    protected final Set predecessors;

    /** The unique ID for the next-created object. */
    private static final AtomicLong nextUid = new AtomicLong(0);

    /** The unique ID of this object. */
    private final transient long uid = nextUid.getAndIncrement();

    @Override
    public long getUid(@UnknownInitialization BlockImpl this) {
        return uid;
    }

    /**
     * Create a new BlockImpl.
     *
     * @param type the type of this basic block
     */
    protected BlockImpl(BlockType type) {
        this.type = type;
        // Most blocks have few predecessors.
        this.predecessors = new ArraySet<>(2);
    }

    @Override
    public BlockType getType() {
        return type;
    }

    @Override
    public Set getPredecessors() {
        // Not "Collections.unmodifiableSet(predecessors)" which has nondeterministic iteration
        // order.
        return new ArraySet<>(predecessors);
    }

    public void addPredecessor(BlockImpl pred) {
        predecessors.add(pred);
    }

    public void removePredecessor(BlockImpl pred) {
        predecessors.remove(pred);
    }
}