Mapreduce中map方法自定义key类

为map方法的key构造一个复合类

public static class IntPair implements WritableComparable<IntPair> {  
        int first;  
        int second;  
        /** 
         * Set the left and right values. 
         */  
        public void set(int left, int right) {  
            first = left;  
            second = right;  
        }  
        public int getFirst() {  
            return first;  
        }  
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