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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.nullness.qual.Nullable;
import org.checkerframework.dataflow.analysis.Store.FlowRule;
import org.checkerframework.dataflow.cfg.node.Node;
import org.checkerframework.javacutil.BugInCF;
import org.plumelib.util.ArraySet;

import java.util.Collections;
import java.util.List;
import java.util.Set;

/** Implementation of a conditional basic block. */
public class ConditionalBlockImpl extends BlockImpl implements ConditionalBlock {

    /** Successor of the then branch. */
    protected @Nullable BlockImpl thenSuccessor;

    /** Successor of the else branch. */
    protected @Nullable BlockImpl elseSuccessor;

    /**
     * The initial value says that the THEN store before a conditional block flows to BOTH of the
     * stores of the then successor.
     */
    protected FlowRule thenFlowRule = FlowRule.THEN_TO_BOTH;

    /**
     * The initial value says that the ELSE store before a conditional block flows to BOTH of the
     * stores of the else successor.
     */
    protected FlowRule elseFlowRule = FlowRule.ELSE_TO_BOTH;

    /**
     * Initialize an empty conditional basic block to be filled with contents and linked to other
     * basic blocks later.
     */
    public ConditionalBlockImpl() {
        super(BlockType.CONDITIONAL_BLOCK);
    }

    /** Set the then branch successor. */
    public void setThenSuccessor(BlockImpl b) {
        thenSuccessor = b;
        b.addPredecessor(this);
    }

    /** Set the else branch successor. */
    public void setElseSuccessor(BlockImpl b) {
        elseSuccessor = b;
        b.addPredecessor(this);
    }

    @Override
    public Block getThenSuccessor() {
        if (thenSuccessor == null) {
            throw new BugInCF(
                    "Requested thenSuccessor for conditional block before initialization");
        }
        return thenSuccessor;
    }

    @Override
    public Block getElseSuccessor() {
        if (elseSuccessor == null) {
            throw new BugInCF(
                    "Requested elseSuccessor for conditional block before initialization");
        }
        return elseSuccessor;
    }

    @Override
    public Set getSuccessors() {
        Set result = new ArraySet<>(2);
        result.add(getThenSuccessor());
        result.add(getElseSuccessor());
        return result;
    }

    @Override
    public FlowRule getThenFlowRule() {
        return thenFlowRule;
    }

    @Override
    public FlowRule getElseFlowRule() {
        return elseFlowRule;
    }

    @Override
    public void setThenFlowRule(FlowRule rule) {
        thenFlowRule = rule;
    }

    @Override
    public void setElseFlowRule(FlowRule rule) {
        elseFlowRule = rule;
    }

    /**
     * {@inheritDoc}
     *
     * This implementation returns an empty list.
     */
    @Override
    public List getNodes() {
        return Collections.emptyList();
    }

    @Override
    public @Nullable Node getLastNode() {
        return null;
    }

    @Override
    public String toString() {
        return "ConditionalBlock()";
    }
}