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Java 中强制执行类实例不变量的最佳方法是什么（即确保某些语句在调用任何公共方法之前和之后都是正确的）？

我将在下面包含一个示例（让我对这个问题感到疑惑的示例），但实际上我对强加类实例不变量的通用解决方案更感兴趣。

假设我有以下课程：

public class Constraint
{
  private int cardinality;

  // constructor and getter/setter omitted
}

public class Edge
{
  private int minCardinality = 1;
  private int maxCardinality = Integer.MAX_VALUE;
  private Constraint constraint = null;

  // constructors, getters/setters and other methods omitted
}

并且我想对 Edge 类强制执行以下语句：

  • 0 <= minCardinality <= maxCardinality
  • if (constraint != null) then (minCardinality <= constraint.getCardinality() <= maxCardinality)

此处 Edge 实例由具有（强制性）属性 minCardinality 和 maxCardinality 集的解析器创建。（可选）约束属性留空。然后将实例移交给另一个模块（测试帧生成器），该模块可能会或可能不会通过 setter 方法设置约束。

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1 回答 1

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使用构建器并将支票放入其中；这样你的Edge班级就不会关心：

@Immutable // See JSR 305
public final class Edge
{
    private final int minCardinality;
    // ...

    public static Builder newBuilder() { return new Builder(); }

    // Edge has no public constructor anymore, only this one:
    private Edge(final Builder builder)
    {
        minCardinality = builder.minCardinality;
        // etc
    }

    @NotThreadSafe // See JSR 305
    public static final class Builder
    {
        private int minCardinality;
        // ...

        private Builder() {}

        public Builder withMinCardinality(final int minCardinality)
        {
            this.minCardinality = minCardinality;
            return this;
        }
        // etc

        public Edge build()
        {
            // checks here
            return new Edge(this);
        }
    }
}

用法：

final Edge edge = Edge.newBuilder().withMinCardinality(xxx).etc().etc().build();
于 2013-06-14T08:43:43.683 回答