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货币，Cointrader中基本实体。我们通过代码来学习该实体：

package org.cryptocoinpartners.schema;

import java.util.List;

import javax.persistence.Cacheable;

import javax.persistence.Entity;

import javax.persistence.NoResultException;

import org.cryptocoinpartners.util.PersistUtil;

/**

 * @author Tim Olson

 */

@Entity //实体

@Cacheable

public class Currency extends Asset {//货币是资产的一种，继承关系

    public boolean isFiat() {//是否为法定货币

        return fiat;

    }

    public static Currency forSymbol(String symbol) {

        return PersistUtil.queryOne(Currency.class, "select c from Currency c where symbol=?1", symbol);

    }

    public static List<String> allSymbols() {

        return PersistUtil.queryList(String.class, "select symbol from Currency");

    }

    // JPA

    protected Currency() {

    }

    protected void setFiat(boolean fiat) {

        this.fiat = fiat;

    }

    // used by Currencies

    static Currency forSymbolOrCreate(String symbol, boolean isFiat, double basis) {

        try {

            return forSymbol(symbol);

        } catch (NoResultException e) {

            final Currency currency = new Currency(isFiat, symbol, basis);

            PersistUtil.insert(currency);

            return currency;

        }

    }

    // used by Currencies

    static Currency forSymbolOrCreate(String symbol, boolean isFiat, double basis, double multiplier) {

        try {

            return forSymbol(symbol);

        } catch (NoResultException e) {

            final Currency currency = new Currency(isFiat, symbol, basis, multiplier);

            PersistUtil.insert(currency);

            return currency;

        }

    }

    private Currency(boolean fiat, String symbol, double basis) {

        super(symbol, basis);

        this.fiat = fiat;

    }

    private Currency(boolean fiat, String symbol, double basis, double multiplier) {

        super(symbol, basis);

        this.fiat = fiat;

        this.multiplier = multiplier;

    }

    private boolean fiat;//是否为法定货币

    private double multiplier;//乘数

}

multiplier乘数是一个比較生疏的一个概念。

它的介绍见wikipedia：http://en.wikipedia.org/wiki/Multiplier_(economics)

还是一个比較难的概念。