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  1. /* package whatever; // don't place package name! */
  2. //http://stackoverflow.com/questions/25523570/how-to-decouple-input-params-for-different-handlers-in-a-filter-chain
  3. import java.util.*;
  4. import java.lang.*;
  5. import java.io.*;
  6.  
  7. class FacetedFilter
  8. {
  9.     interface IFilter {
  10.     	boolean apply(Context context);
  11.     }
  12.  
  13. 	/** Unique identifier for context data  **/
  14.     public static class FacetKey<T> {
  15. 		private final Class<T> type;
  16. 		public FacetKey(Class<T> type) {
  17. 			assert type != null;
  18. 			this.type = type;
  19. 		}
  20.  
  21. 		/**No one should override equals of key **/
  22. 		@Override
  23. 		public final boolean equals(Object object) {
  24. 			return super.equals(object);
  25. 		}
  26.     }
  27.  
  28.     /** Holds typed data.
  29.      * Only key holder can access data adressed by that key.
  30.      **/
  31.     public static class Context {
  32. 	    private final Map<FacetKey, Object> data = new HashMap<>();
  33.     	public <T> T put(FacetKey<T> key, T datum) {
  34.     		return key.type.cast(data.put(key, datum));
  35.     	}
  36.     	public <T> T get(FacetKey<T> key){
  37.     		return key.type.cast(data.get(key));
  38.     	}
  39.     }
  40.  
  41.     /** Basic helper for typesafe filter implementation **/
  42.     static abstract class TypedFilter<T> implements IFilter {
  43.     	public final FacetKey<T> key;
  44.     	public TypedFilter(FacetKey<T> key) {
  45.     		this.key = key;
  46.     	}
  47.     	@Override
  48.     	public final boolean apply(Context context) {
  49.     		return apply(context.get(key));
  50.     	}
  51.     	public abstract boolean apply(T datum);
  52.     }
  53.  
  54.     /** Type of data for first filter */
  55. 	static class DataA {
  56. 		int x = 5;
  57. 	}
  58.  
  59.     /** First filter */
  60.     static class FilterA extends TypedFilter<DataA> {
  61.     	/**Each filter has its own key, unknown to other filters.
  62.     	 * This can be made as hidden as needed to prevent key reuse.
  63.     	**/
  64.     	public static final FacetKey<DataA> key = new FacetKey<>(DataA.class);
  65. 		public FilterA() {
  66. 			super(key);
  67. 		}
  68. 		@Override
  69. 		public boolean apply(DataA datum) {
  70. 			return datum.x == 6; // arbitrary predicate
  71. 		}
  72.     }
  73.  
  74. 	static class DataB {
  75. 		String y = "";
  76. 	}
  77.  
  78.  
  79.     static class FilterB extends TypedFilter<DataB> {
  80.     	/**Each filter has its own key, unknown to other filters.
  81.     	 * This can be made as hidden as needed to prevent key reuse.
  82.     	**/
  83.     	public static final FacetKey<DataB> key = new FacetKey<>(DataB.class);
  84. 		public FilterB() {
  85. 			super(key);
  86. 		}
  87. 		@Override
  88. 		public boolean apply(DataB datum) {
  89. 			return datum.y != null; // another predicate
  90. 		}
  91.     }
  92.  
  93.  
  94.     static class FilterC extends TypedFilter<DataB> {
  95.     	//Data type is not unique, but key is.
  96.     	//This key won't let you access B filter data.
  97.     	public static final FacetKey<DataB> key = new FacetKey<>(DataB.class);
  98. 		public FilterC() {
  99. 			super(key);
  100. 		}
  101. 		@Override
  102. 		public boolean apply(DataB datum) {
  103. 			return java.util.Objects.equals(datum.y, "Hey!"); // another predicate
  104. 		}
  105.     }
  106.  
  107.     static void println(Object line) {
  108.     	System.out.println(""+line);
  109.     }
  110.  
  111. 	public static void main (String[] args) throws java.lang.Exception
  112. 	{
  113. 		Context context = new Context();
  114. 		context.put(FilterA.key, new DataA());
  115. 		context.put(FilterB.key, new DataB());
  116. 		DataB c = new DataB();
  117. 		c.y = "Hey!";
  118. 		context.put(FilterC.key, c);
  119. 		println(new FilterA().apply(context));
  120. 		println(new FilterB().apply(context));
  121. 		println(new FilterC().apply(context));
  122. 	}
  123. }
Success #stdin #stdout 0.07s 380160KB
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https://ideone.com/TftRsG
language:
Java (12.0.1)
created:
11 years ago
visibility:
public

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