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Setting up partitionExprs in Apache Spark using Java involves the following steps:

1. Create a List of expression objects: partitionExprs is a List of expressions used to partition the input data. You need to create a List of expression objects to define the partitioning criteria for your data.

2. Define the expressions: The expression objects can be created using various methods of the Expression class. For example, you can use the col method to create an expression from a column name or the expr method to create an expression from a SQL expression.

3. Set up the partitioning: Finally, you can set up the partitioning criteria using the partitionBy method of the Dataset class. This method takes in the List of expression objects as a parameter and partitions the data based on the specified criteria.

Here's an example code snippet in Java that demonstrates the process of setting up partitionExprs:

import org.apache.spark.sql.Dataset;
import org.apache.spark.sql.Encoders;
import org.apache.spark.sql.Row;
import org.apache.spark.sql.SparkSession;
import org.apache.spark.sql.functions;

import java.util.ArrayList;
import java.util.List;

public class PartitioningExample {

    public static void main(String[] args) {

        SparkSession spark = SparkSession.builder()
                .appName("PartitioningExample")
                .master("local[*]")
                .getOrCreate();

        // create a sample dataset
        List<String> data = new ArrayList<>();
        data.add("John,Doe,25");
        data.add("Jane,Doe,30");
        data.add("Bob,Smith,40");
        Dataset<String> dataset = spark.createDataset(data, Encoders.STRING());

        // create a list of expressions to partition the dataset by age
        List<org.apache.spark.sql.Column> partitionExprs = new ArrayList<>();
        partitionExprs.add(functions.col("age"));

        // partition the dataset by age
        Dataset<Row> partitionedDataset = dataset
                .select(functions.split(functions.col("value"), ",").getItem(0).as("first_name"),
                        functions.split(functions.col("value"), ",").getItem(1).as("last_name"),
                        functions.split(functions.col("value"), ",").getItem(2).cast("int").as("age"))
                .drop("value")
                .repartition(partitionExprs.toArray(new org.apache.spark.sql.Column[0]));

        // show the partitioned dataset
        partitionedDataset.show();
    }
}

In the example above, we create a sample dataset and then create a List of expression objects to partition the dataset by age. We then use the partitionBy method to partition the dataset by the specified criteria. Finally, we show the partitioned dataset using the show method.