目前我正在使用以下代码在 jpa 中进行一些过滤:

if (value.getClass() == Integer.class) {
   return cb.greaterThan(root.<Integer>get(field), (Integer) value);
} else if (value.getClass() == Long.class) {
   return cb.greaterThan(root.<Long>get(field), (Long) value);
} else if (value.getClass() == Float.class) {
   return cb.greaterThan(root.<Float>get(field), (Float) value);
} else if (value.getClass() == Date.class) {
   return cb.greaterThan(root.<Date>get(field), (Date) value);
}

我怎样才能将这个 block 减少到这样的一行？

return cb.greaterThan(root.<value.getClass()>get(field), value);

所以我需要用我的类类型替换 < t=""> 中的 T 值。可悲的是，我在 Java 泛型方面不是那么好。有人有想法吗？有可能吗？

root 属于以下类型:http://docs.oracle.com/javaee/6/api/javax/persistence/criteria/Path.html#get%28java.lang.String%29

编辑:这是我要写的完整类(class):

import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.Predicate;
import javax.persistence.criteria.Root;

public class FilterExpression {

    public static final Integer BEGINS_WITH = 0;
    public static final Integer ENDS_WITH = 1;
    public static final Integer CONTAINS = 2;
    public static final Integer EQUAL = 3;
    public static final Integer NOT_EQUAL = 4;
    public static final Integer GREATER_THAN = 5;
    public static final Integer GREATER_EQUAL_THAN = 6;
    public static final Integer LESS_THAN = 7;
    public static final Integer LESS_EQUAL_THAN = 8;
    private static final Map<String, Integer> OPERATOR_MAPPING;

    static {
        Map<String, Integer> temp = new HashMap<>();
        temp.put("bw", BEGINS_WITH);
        temp.put("ew", ENDS_WITH);
        temp.put("ct", CONTAINS);
        temp.put("eq", EQUAL);
        temp.put("nq", NOT_EQUAL);
        temp.put("gt", GREATER_THAN);
        temp.put("gq", GREATER_EQUAL_THAN);
        temp.put("lt", LESS_THAN);
        temp.put("lq", LESS_EQUAL_THAN);
        OPERATOR_MAPPING = Collections.unmodifiableMap(temp);
    }

    private String field;
    private Integer operator;
    private Object value;

    public FilterExpression(String field, String operator, String value, Class c) {
        this.field = field;
        setOperator(operator);
        setValue(value, c);
    }

    public Boolean validate() {
        if (StringUtils.isEmpty(field) || operator == null || value == null) {
            return false;
        }
        Class c = value.getClass();

        if (c == String.class) {
            return operator >= BEGINS_WITH && operator <= NOT_EQUAL;
        } else if (c == Integer.class || c == Float.class || c == Double.class) {
            return (EQUAL >= EQUAL && operator <= LESS_EQUAL_THAN);
        } else if (c == Boolean.class) {
            return operator == EQUAL || operator == NOT_EQUAL;
        } else if (c == Identification.Type.class) {
            return operator == EQUAL || operator == NOT_EQUAL;
        }
        return false;
    }

    public String getField() {
        return field;
    }

    public void setField(String field) {
        this.field = field;
    }

    public Integer getOperator() {
        return operator;
    }

    public void setOperator(String operator) {
        this.operator = OPERATOR_MAPPING.get(operator.toLowerCase());
    }

    public Object getValue() {
        return value;
    }

    public void setValue(String s, Class c) {
        try {
            if (Boolean.class == c) {
                this.value = validateBoolean(s);
            } else if (Integer.class == c) {
                this.value = Integer.parseInt(s);
            } else if (Float.class == c) {
                this.value = Float.parseFloat(s);
            } else if (Identification.Type.class == c) {
                this.value = Identification.Type.parse(Integer.parseInt(s));
            } else {
                this.value = s;
            }
        } catch (NumberFormatException ex) {
            this.value = null;
        }
    }

    public <R> Predicate toPredicate(CriteriaBuilder cb, Root<R> root) {
        if (Objects.equals(operator, FilterExpression.EQUAL)) {
            return cb.equal(root.get(field), value);
        } else if (Objects.equals(operator, FilterExpression.NOT_EQUAL)) {
            return cb.notEqual(root.get(field), value);
        } else if (Objects.equals(operator, FilterExpression.CONTAINS)) {
            return cb.like(root.<String>get(field), "%" + value + "%");
        } else if (Objects.equals(operator, FilterExpression.ENDS_WITH)) {
            return cb.like(root.<String>get(field), "%" + value);
        } else if (Objects.equals(operator, FilterExpression.BEGINS_WITH)) {
            return cb.like(root.<String>get(field), value + "%");
        } else if (Objects.equals(operator, FilterExpression.GREATER_THAN)) {
            if (value.getClass() == Integer.class) {
                return cb.greaterThan(root.<Integer>get(field), (Integer) value);
            } else if (value.getClass() == Float.class) {
                return cb.greaterThan(root.<Float>get(field), (Float) value);
            } else if (value.getClass() == Date.class) {
                return cb.greaterThan(root.<Date>get(field), (Date) value);
            }
        } else if (Objects.equals(operator, FilterExpression.GREATER_EQUAL_THAN)) {
            if (value.getClass() == Integer.class) {
                return cb.greaterThanOrEqualTo(root.<Integer>get(field), (Integer) value);
            } else if (value.getClass() == Float.class) {
                return cb.greaterThanOrEqualTo(root.<Float>get(field), (Float) value);
            } else if (value.getClass() == Date.class) {
                return cb.greaterThanOrEqualTo(root.<Date>get(field), (Date) value);
            }
        } else if (Objects.equals(operator, FilterExpression.LESS_THAN)) {
            if (value.getClass() == Integer.class) {
                return cb.lessThan(root.<Integer>get(field), (Integer) value);
            } else if (value.getClass() == Float.class) {
                return cb.lessThan(root.<Float>get(field), (Float) value);
            } else if (value.getClass() == Date.class) {
                return cb.lessThan(root.<Date>get(field), (Date) value);
            }
        } else if (Objects.equals(operator, FilterExpression.LESS_EQUAL_THAN)) {
            if (value.getClass() == Integer.class) {
                return cb.lessThanOrEqualTo(root.<Integer>get(field), (Integer) value);
            } else if (value.getClass() == Float.class) {
                return cb.lessThanOrEqualTo(root.<Float>get(field), (Float) value);
            } else if (value.getClass() == Date.class) {
                return cb.lessThanOrEqualTo(root.<Date>get(field), (Date) value);
            }
        }
        return null;
    }

最佳答案

我猜你可以让整个类通用。

public class FilterExpression<T extends Comparable<T>> {
    //Make value to be type T instead of Object.
    T value;

    // And you could easily do,
    ...
    return cb.greaterThan(root.<T>get(field), value);
}

关于Java 泛型变量 <T> 值，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/39000071/