如何忽略空值？

Question

在我的数据库中，我有 4 个数据字段，我还有一个方法 (getWalletName)，我发送一个 SQL 查询来只检索 4 个数据中的一个，但是之后检索并显示结果。我可以像这样看到所需数据旁边的其他数据：

SELECT Name FROM wallets WHERE Location ='ROOME' ;

[unknown,unknown,unknown,Omar]

我希望它像那样显示的地方 [Omar]，请记住不要修改我的构造函数，因为我需要它们以这种方式显示。我试过下面 wallet class 中的代码，但没有用！任何帮助将不胜感激

wallet class :

public class wallet {
private String Name;
private String Location;
private String Tag;
private String Reader;


public wallet(String Name , String Location, String Tag, String Reader)
{       
    this.Name = Name;
    this.Location = Location;
    this.Tag = Tag; 
    this.Reader = Reader;   

}


public wallet(String Reader)
{       
    this.Reader = Reader;
    this.Name = "unknown";
    this.Location = "unknown";
    this.Tag = "unknown";   
}

public wallet(String Location,String Tag) {
    this.Location = Location;
    this.Tag = Tag;
}

public String getWalletName()
{
    return Name;
}
public void setWalletName(String Name)
{
    this.Name = Name;
}

public String getWalletLocation()
{
    return Location;
}
public void setWalletLocation(String Location)
{
    this.Location = Location;
}

public String getWalletTag()
{
    return Tag;
}
public void setWalletTag(String Tag)
{
    this.Tag = Tag;
}


@Override
public String toString()
{
     StringBuilder sb = new StringBuilder();

        if(Objects.nonNull(this.Name)) {
            sb.append(this.Name+",");
        }
        if(Objects.nonNull(this.Location)) {
            sb.append(this.Location+",");
        }
        if(Objects.nonNull(this.Tag)) {
            sb.append(this.Tag+",");
        }
        if(Objects.nonNull(this.Reader)) {
            sb.append(this.Reader);
        }

        return sb.toString();
}

}

getWalletName method:

public ArrayList<wallet> getWalletName(String Location) throws SQLException {
    System.out.println("----------------");
    System.out.println("Retrieve the wallet name");

    //Connection dbConnection = null;
    //Statement statement = null;
    ResultSet resultset = null;     
    String query = "SELECT Name FROM wallets WHERE Location ='" + Location + "' ;";
    ArrayList<wallet> WalletsList = new ArrayList<>();      
    try {
        getConnection();
        //statement = dbConnection.createStatement();
        System.out.println(query);
        resultset = stmt.executeQuery(query);

        while (resultset.next()) {

            String Name =  resultset.getString("Name");
            WalletsList.add(new wallet (Name));
        }
    } 
    finally {

    //  closeConnection();

        if (resultset !=null){
            resultset.close();
        }
        if (stmt !=null) {
            stmt.close();
        }

        if (resultset !=null) {
            closeConnection();
        }
    }
    return WalletsList;

}

Answer 1

把这个放在下面String Name = resultset.getString("Name");，应该可以

If resultset.getString("Name") != null {
    WalletsList.add(new wallet (Name));
}

Answer 2

如果你不想改变你的构造函数，只有一个办法：

class wallet {
    private String Name;
    private String Location;
    private String Tag;
    private String Reader;


    public wallet(String Name, String Location, String Tag, String Reader) {
        this.Name = Name;
        this.Location = Location;
        this.Tag = Tag;
        this.Reader = Reader;

    }


    public wallet(String Reader) {
        this.Reader = Reader;
        this.Name = "unknown";
        this.Location = "unknown";
        this.Tag = "unknown";
    }

    public wallet(String Location, String Tag) {
        this.Location = Location;
        this.Tag = Tag;
    }

    public String getWalletName() {
        return Name;
    }

    public void setWalletName(String Name) {
        this.Name = Name;
    }

    public String getWalletLocation() {
        return Location;
    }

    public void setWalletLocation(String Location) {
        this.Location = Location;
    }

    public String getWalletTag() {
        return Tag;
    }

    public void setWalletTag(String Tag) {
        this.Tag = Tag;
    }


    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();

        if (isPrintableValue(this.Name)) {
            sb.append(this.Name + ",");
        }
        if (isPrintableValue(this.Location)) {
            sb.append(this.Location + ",");
        }
        if (isPrintableValue(this.Tag)) {
            sb.append(this.Tag + ",");
        }
        if (isPrintableValue(this.Reader)) {
            sb.append(this.Reader);
        }

        return sb.toString();
    }

    protected boolean isPrintableValue(String value) {
        return (value != null && !value.equals("unknown"));
    }
}

public class Test {

    public static void main(String[] args) {
        wallet wallet = new wallet("Omar");
        System.out.println(wallet);
    }
}

输出：

Omar

Answer 3

我建议 toString() 钱包 class 的替代版本，也许更紧凑：

public String toString() {
    StringBuilder sb = new StringBuilder();

    sb.append("[");
    sb.append(checkValue(this.Name,","));
    sb.append(checkValue(this.Location,","));
    sb.append(checkValue(this.Tag,","));
    sb.append(checkValue(this.Reader,""));
    sb.append("]");

    return sb.toString();
}

protected String checkValue(String value, String separator) {
    return (value != null && !value.equals("unknown")) ? (value+separator) : "";
}

此外，首先我建议使用CamelCase来编写方法、变量、classes、包和常量的名称：这是一种标准的表示法（你不必强制，它的工作原理是一样的，但是对于开发人员社区来说更容易理解）。所以最好每个单词或缩写都以大写字母开头，或者第一个单词以小写字母开头，其余全部大写。其次，我建议用户使用 PreparedStatement 而不是简单的 Statement，因为出于安全原因更可取：动态构建 Statement 容易受到 SQL 注入的攻击（使用 PreparedStatement 可以避免在谓词中引用值时出现的语法错误）。

所以代替：

String query = "SELECT Name FROM wallets WHERE Location ='" + Location + "' ;";

您可以尝试使用：

String query = "SELECT Name FROM wallets WHERE Location = ? ";

连同类似的东西（假设 conn 是你的数据库连接）：

try {
    String query = "SELECT Name FROM wallets WHERE Location = ? ";
    try (PreparedStatement stmt = conn.prepareStatement(query); ResultSet resultset = stmt.executeQuery(query)) {
        while (resultset.next()) {
            String Name = resultset.getString("Name");
            WalletsList.add(new wallet(Name));
        }
    }
} catch (SQLException sQLException) {
    sQLException.printStackTrace();
}

请注意，我使用 "try-with-resource" 语法来避免关闭结果集和语句的问题（即带有 resultset.close()、stament.close() 等的 finally 子句这是冗长的，如果面对不正确，有时可能会导致一些编码错误。

最好的是

如何忽略空值？

How do I ignore the null values?

java

mysql

sql

tostring