Python工程师Java之路（h）Java字符串拼接+null拼接处理

Java版本：1.8

加号

String a = "ab" + 'c' + 5L;
System.out.println(a);
//abc5

String b = 9.99F + "ABC" + null;
System.out.println(b);
//9.99ABCnull
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String的concat

System.out.println("aa".concat("bb"));
//aabb
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StringBuilder和StringBuffer的append

StringBuilder stringBuilder = new StringBuilder("ab");
stringBuilder.append(99L).append("cd");
System.out.println(stringBuilder);
//ab99cd

StringBuffer stringBuffer = new StringBuffer("ab");
stringBuffer.append(9.99F).append("cd");
System.out.println(stringBuffer);
//ab9.99cd
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StringBuffer是线程安全的（synchronized），源码截取如下

    @Override
    synchronized StringBuffer append(AbstractStringBuilder asb) {
        toStringCache = null;
        super.append(asb);
        return this;
    }
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StringBuilder的append不是线程安全的，速度更快，源码截取如下

    public StringBuilder append(StringBuffer sb) {
        super.append(sb);
        return this;
    }
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String的join

String a = String.join("-", "a", "b", "c");
System.out.println(a);
//a-b-c

String[] strings = {"a", "b", "c", null, "e"};
String b = String.join("-", strings);
System.out.println(b);
//a-b-c-null-e
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源码截取如下，第一个参数为分隔符

    public static String join(CharSequence delimiter, CharSequence... elements) {
        Objects.requireNonNull(delimiter);
        Objects.requireNonNull(elements);
        // Number of elements not likely worth Arrays.stream overhead.
        StringJoiner joiner = new StringJoiner(delimiter);
        for (CharSequence cs: elements) {
            joiner.add(cs);
        }
        return joiner.toString();
    }
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StringJoiner

StringJoiner实例化时，可设置(分隔符,前缀,后缀)

import java.util.StringJoiner;

public class Hi {
    public static void main(String[] args) {
        //StringJoiner(分隔符, 前缀, 后缀)
        StringJoiner stringJoiner = new StringJoiner(",", "[", "]");
        //遍历
        String[] strings = {"a", "b", "c", null, "e"};
        for (String string : strings) {
            //过滤空值
            if (string != null) {
                stringJoiner.add(string);
            }
        }
        //打印
        System.out.println(stringJoiner.toString());
    }
}
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打印效果

[a,b,c,e]

StringUtils.join

import com.sun.deploy.util.StringUtils;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Objects;

public class Hi {
    public static void main(String[] args) {
        ArrayList<String> a = new ArrayList<>(Arrays.asList("a", "b", null, "d"));
        //不过滤空值
        System.out.println(StringUtils.join(a, ",")); //a,b,null,d
        //过滤空值
        a.removeIf(Objects::isNull);
        System.out.println(StringUtils.join(a, ",")); //a,b,d
    }
}
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场景：拼接SQL

Phoenix（待完善）

import com.alibaba.fastjson.JSONObject;
import com.sun.deploy.util.StringUtils;

public class Hello {
    public static void main(String[] args) {
        String s = "{\"tb\":\"sku\",\"data\":{\"id\":\"99\",\"name\":\"椰子\"}}";
        JSONObject js = JSONObject.parseObject(s);
        String table = js.getString("tb");
        JSONObject data = js.getJSONObject("data");
        String sql = "upsert into " + table
                + " (" + String.join(",", data.keySet()) + ") "
                + "values"
                + " ('" + StringUtils.join(data.values(), "','") + "');";
        System.out.println(sql);
    }
}
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打印结果

upsert into sku (name,id) values ('椰子','99');
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MySQL（待完善）

import com.alibaba.fastjson.JSONObject;

import java.util.StringJoiner;

public class Hi {
    public static void main(String[] args) {
        String s = "{\"tb\":\"sku\",\"data\":{\"id\":99,\"name\":\"椰子\"}}";
        JSONObject js = JSONObject.parseObject(s);
        String table = js.getString("tb");
        JSONObject data = js.getJSONObject("data");
        StringJoiner columns = new StringJoiner(",", "(", ")");
        StringJoiner values = new StringJoiner(",", "(", ")");
        for (String column : data.keySet()) {
            columns.add(column);
            Object value = data.get(column);
            if (value.getClass() == String.class) {
                values.add("'" + value + "'");
            } else {
                values.add(value.toString());
            }
        }
        System.out.println("INSERT " + table + columns + " VALUES " + values + ";");
    }
}
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打印结果

INSERT sku(name,id) VALUES ('椰子',99);
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