java - Java泛型-不兼容的类型

只需要一些有关此错误的代码帮助：

interface ParticleId{
    int getX();
}

class ParticleId1 implements ParticleId{
    int a;
    @Override
    public int getX() {
        return a;
    }
}

class ParticleId2 implements ParticleId{
    int a,b;
    @Override
    public int getX() {
        return a+b;
    }
}

interface ParticleInt<K extends ParticleId, O extends ParticleInt<K, O>>{
    O withSmt();
}

class Particle<K extends ParticleId> implements ParticleInt<K, Particle<K>>{
    @Override
    public Particle<K> withSmt() {
        return new Particle<>();
    }
}

class Particle2<K extends ParticleId> extends Particle<K>{
    @Override
    public Particle2<K> withSmt() {
        return new Particle2<>();
    }
}

interface Executable<P extends Particle<? extends ParticleId>>{
    void setExecutableStrategy(ExecutableStrategy<P> s);
}

interface ExecutableStrategy<P extends Particle<? extends ParticleId>>{
    Stream<P> exec(List<P> l);
}

class Particle2Strat implements ExecutableStrategy<Particle2<? extends ParticleId>>{
    @Override
    public Stream<Particle2<? extends ParticleId>> exec(List<Particle2<? extends ParticleId>> l) {
        return l.stream().map(x -> x.withSmt());
    }
}

class ParticleStrat<P extends Particle<? extends ParticleId>> implements ExecutableStrategy<P>{
    @Override
    public Stream<P> exec(List<P> l) {
        return l.stream().map(x -> x.withSmt());
        // [44,38] incompatible types: inference variable R has incompatible bounds
        // equality constraints: P
        // [ERROR] lower bounds: Particle<capture#1 of ? extends ParticleId>
    }
}

该错误显示在代码末尾，确切描述为[44,38] incompatible types: inference variable R has incompatible bounds. equality constraints: P. [ERROR] lower bounds: Particle<capture#1 of ? extends ParticleId>。

我发现有效的快速修复方法是手动将lambda强制转换为（P），例如(P)x.withSmt()。如果有可能，有人知道更正确的方法来解决此问题吗？

我刚刚更新了代码，因此它不再使用Integer而是ParticleId接口。

更新

更新后，我现在遇到这个问题：

class ParticleStrat<P extends ParticleId> implements ExecutableStrategy<Particle<P>>{
    @Override
    public Stream<Particle<P>> exec(List<Particle<P>> l) {
        return l.stream().map(x -> x.withSmt());  //OK NOW
    }
}

class MainExecutable<P extends Particle<? extends ParticleId>> implements Executable<P>{

    private ExecutableStrategy<P> s;

    public MainExecutable() {
        this.s = new ParticleStrat<>();
        //incompatible types: cannot infer type arguments for ParticleStrat<>
        //no instance(s) of type variable(s) P exist so that ParticleStrat<P> conforms to ExecutableStrategy<P>
    }

    @Override
    public void setExecutableStrategy(ExecutableStrategy<P> s) {
        this.s = s;
    }
}

最佳答案

将ParticleStrat类的声明更改为以下内容可以解决编译错误：

class ParticleStrat<P extends ParticleInt<P>> implements ExecutableStrategy<P>{
    @Override
    public Stream<P> exec(List<P> l) {
        return l.stream().map(x -> x.withSmt());
    }
}

这样，保证x.withSmt()返回P。

根据您的问题更改进行编辑：

通过编译：

class ParticleStrat<K extends Integer> implements ExecutableStrategy<Particle<K>>{
    @Override
    public Stream<Particle<K>> exec(List<Particle<K>> l) {
        return l.stream().map(x -> x.withSmt());
    }
}

在最新更新之后：

class ParticleStrat<K extends ParticleId> implements ExecutableStrategy<Particle<K>>{
    @Override
    public Stream<Particle<K>> exec(List<Particle<K>> l) {
        return l.stream().map(x -> x.withSmt());
    }
}

对于您的最新问题，可以使用相同的方法：

class MainExecutable<K extends ParticleId> implements Executable<Particle<K>>{

    private ExecutableStrategy<Particle<K>> s;

    public MainExecutable() {
        this.s = new ParticleStrat<>();
    }

    @Override
    public void setExecutableStrategy(ExecutableStrategy<Particle<K>> s) {
        this.s = s;
    }
}