How to get the output from a Stack after creating it?

Here is an example for Java SDK

Modify your stack to include a CfnOutput for the Load Balancer's DNS name:

package com.myorg;

import software.amazon.awscdk.core.*;
import software.amazon.awscdk.services.ec2.Vpc;
import software.amazon.awscdk.services.ecs.Cluster;
import software.amazon.awscdk.services.ecs.patterns.ApplicationLoadBalancedFargateService;
import software.amazon.awscdk.services.ecs.ContainerImage;

public class FargateStack extends Stack {
    public FargateStack(final Construct scope, final String id, final StackProps props) {
        super(scope, id, props);

        Vpc vpc = Vpc.Builder.create(this, "MyVpc").maxAzs(3).build();
        Cluster cluster = Cluster.Builder.create(this, "MyCluster").vpc(vpc).build();

        ApplicationLoadBalancedFargateService fargateService = ApplicationLoadBalancedFargateService.Builder.create(this, "MyFargateService")
                .cluster(cluster)
                .cpu(256)
                .desiredCount(1)
                .taskImageOptions(
                        ApplicationLoadBalancedTaskImageOptions.builder()
                                .image(ContainerImage.fromRegistry("amazon/amazon-ecs-sample"))
                                .build())
                .memoryLimitMiB(512)
                .publicLoadBalancer(true)
                .build();

        // Export the Load Balancer DNS name
        CfnOutput.Builder.create(this, "ServiceUrl")
                .exportName("MyFargateServiceUrl") // This is the key name you will use to import in other stacks
                .value(fargateService.getLoadBalancer().getLoadBalancerDnsName())
                .build();
    }
}

public class MyApp extends App {
    public static void main(final String[] args) {
        App app = new App();
        new FargateStack(app, "MyFargateStack", new StackProps.builder().build());
        app.synth();
    }
}

Use the Exported URL in Another Service or Stack

import software.amazon.awscdk.core.Fn;

public class AnotherServiceStack extends Stack {
    public AnotherServiceStack(final Construct scope, final String id, final StackProps props) {
        super(scope, id, props);

        // Import the URL
        String importedUrl = Fn.importValue("MyFargateServiceUrl");

        // Use the imported URL here for whatever needs it
        System.out.println("Imported Service URL: " + importedUrl);
    }
}