Java convert object to StringArray and iterate the array to print values-CodePudding

I have a POJO as follows:

public class TestDto{
  private String columnA;
  private String columnB;
  private String columnC;
  private String columnD;
  ...//...//
}

DB returns me a list of TestDto;

List<TestDto> dto = //resultsList from DB.;

The parameters in TestDto may be columnA,columnB,columnC,columnD,......columnCE,columnCF....it's dynamic.

I want to convert list of TestDto to list of String array using lambda and then iterate the list and string array and assign the values to cell in excel.

I tried below but this didnt work.

List<String> stringList = dto.stream().map(Object::toString).collect(Collectors.toList());

for(int i=0; i<stringList.size(); i  )
{
   System.out.println("stringlist..." stringList.get(i));
}

The output that gets printed is TestDto object but not string[]. I want to convert List<TestDto> to List<String[]> and then do the foll.

List<TestDto> listTestDto =...;
String[] stringArray = null;
    
    for(int i=0; i<listTestDto.size(); i  )
    {
     stringArray = listTestDto.get(i);
     for(int j=0; j< stringArray.length; j  ){
     System.out.println("stringArray - " stringArray[i]);
     }
    }

Data: List:

columnA:1, columnB:ABC, columnC: 11111, columnD :null.....
columnA:2, columnB:DEF, columnC: 22222, columnD :null.....
columnA:3, columnB:GHI, columnC: 33333, columnD :null.......
columnA:4, columnB:JKL, columnC: 44444, columnD :null......

Output:

stringArray - 1
stringArray - ABC
stringArray - 11111
stringArray - null
stringArray - 2
stringArray - DEF
stringArray - 22222
stringArray - null
stringArray - 3
stringArray - GHI
stringArray - 33333
stringArray - null
stringArray - 4
stringArray - JKL
stringArray - 44444
stringArray - null

USeCase:

1. output is List<TestDto>
2. output is List<Object[]>

CodePudding user response：

You need a transformer method toStringArray for TestDto

Add toStringArray method to TestDto

public class TestDto {
    private String columnA;
    private String columnB;
    private String columnC;
    private String columnD;
    ...//...//
    
    public String[] toStringArray() {
        return new String[] { columnA, columnB, columnC, columnD,... };
    }
}

Use map operator from stream and pass transformer method reference

List<String[]> output = list.stream().map(TestDto::toStringArray).collect(Collectors.toList());

Test your output

// Test output
for (int i = 0; i < output.size(); i  ) {
    System.out.println(Arrays.toString(op.get(i)));
}

CodePudding user response：

If TestDto has dynamic fields, that is, its structure is undefined, and the entire TestDto can be represented as a map Map<String, String>.

Therefore, the list of TestDto is changed into the list of maps List<Map<String, String>>, which can be transformed into List<String[]> where each string array stores map values:

List<Map<String, String>> listTestDto =...; // 

List<String[]> strings = listTestDto
    .stream() // Stream<Map<String, String>>
    .map(Map::values) // Stream<Collection<String>>
    .map(v -> v.toArray(new String[0]))
    .collect(Collectors.toList());